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THE 

REFRACTION 

OP 

THE    EYE 

A     MANUAL    FOB    STUDENTS 

BY 

GUSTAVUS    HARTRIDGE,   F.R.C.S. 

SENIOR  SURGEON  TO  THE  ROYAL  WESTMINSTER  OPHTHALMIC  HOSPITAL 

OPHTHALMIC  SURGEON  AND  LECTURER  ON  OPHTHALMIC  SURGERY  TO 

THE  WESTMINSTER  HOSPITAL  ;  CONSULTING  OPHTHALMIC  SURGEON 

TO  ST.  Bartholomew's  hospital,  Chatham,  and  to  st. 
George's  dispensary,  hanoyer  square,  etc. 

WITH  ONE  HUNDRED  AND  NINE  ILLUSTRATIONS 

FOURTEENTH    EDITION 


PHILADELPHIA 
P.    BLAKISTON^S   SON   &   CO. 

1012,  WALNUT  STEEET 
1907 


OPTO 


First      Edition,   Jan.,      1884 


Second           „ 

„ 

1886 

Third 

» 

1888 

Fourth          „ 

Nov., 

1889 

Fifth              „ 

July, 

1891 

Sixth             „ 

Oct., 

1892 

Seventh        „ 

Sept., 

1894 

Eighth 

June, 

1896 

Ninth             „ 

Aug., 

1898 

Tenth 

March,  1900 

Eleventh     „ 

July, 

1901 

Twelfth       „ 

Oct., 

1903 

Thirteenth  „ 

June, 

1905 

Fourteenth,, 

Jan., 

1907 

Total  28,000  copies. 


PRINTED   IN    GREAT   BRITAIN. 


PREFACE    QpTO 


FOURTEENTH     EDITION 


# 


In  preparing  tlie  f ourteentli  edition  of  '  Refraction 
of  the  Eye '  for  publication,  tlie  original  plan  of  tlie 
book  has  been  maintained,  and  no  effort  has  been 
spared  to  make  the  work  more  worthy  of  the  favour 
with  which  it  has  been  received  in  this  country  and 
abroad. 

Although  but  a  short  time  has  elapsed  since  the 
last  edition  of  3000  was  published,  the  book  has 
been  carefully  revised,  and  alterations  made  in 
accordance  with  our  increasing  knowledge  of  the 
subject. 

a.  H. 

12,  WiMPOLE  Street,  W. 
January,  1907. 


PREFACE 


FIRST     EDITION 


I  HAVE  endeavoured  in  the. following  pages  to  state 
"briefly  and  clearly  the  main  facts  with  which  practi- 
tioners and  students  should  be  acquainted,  in  order 
to  enable  them  to  diagnose  errors  of  refraction  accu- 
rately, and  to  prescribe  suitable  glasses  for  their 
correction. 

Those  who  would  do  this  with  facility  can  only 
acquire  the  requisite  amount  of  dexterity  by  prac- 
tically working  out  a  large  number  of  cases  of  refrac- 
tion. No  book,  or  even  the  knowledge  gained  by 
watching  others  who  are  thus  employed,  can  take  the 
place  of  this,  the  practical  part  of  the  subject. 

To  many  of  my  readers  the  chapter  on  Optics  may 
appear  unnecessary.  I  have  added  it  for  the  benefit 
of  those  whose  school  education  did  not  include  this 
subject,  since  its  elementary  details  so  completely 
underlie  the  whole  subject  of  refraction,  that  every 


VI  PREFACE 

student   should  understand  tliem  thoroughly  before 
passing  on  to  the  real  subject  in  hand. 

I  have  found  it  necessary  in  several  instances  to 
repeat  important  matters,  and  this  I  have  done  to 
obviate  the  necessity  of  continual  reference  to  other 
parts  of  the  book,  as  well  as  in  some  cases  to  impress 
the  importance  of  the  subject  upon  the  student. 

The  woodcuts  are  numerous  in  proportion  to  the 
size  of  the  work,  but  I  consider  that  they  are  a  very 
great  help  to  the  thorough  understanding  of  the 
subject. 

The  old  measurements  have  been  purposely  omitted 
in  favour  of  the  almost  universally  adopted  metrical 
system.  It  is  confusing  to  the  learner  to  have  two 
distinct  sets  of  measurements  to  deal  with,  and  no 
possible  good  can  accrue  from  perpetuating  the  old 
system  of  feet  a.nd  inches. 

At  the  end  of  the  work  I  have  given  a  list  of  those 
authors  to  whom  I  have  been  indebted  for  much 
valuable  information  ;  and  in  conclusion,  I  take  this 
opportunity  of  thanking  my  numerous  friends  for 
their  help  and  suggestions. 

G.  H. 

January,  1884. 


CONTENTS 


CHAPTER  I 

PAGE 

Optics    .  .  .  ...  .1 

Reflection        .  .  .  .  .2 

Refraction       .  .  .  .  .6 

Formation  of  Images  .  .  .  .17 


CHAPTER  II 

Refraction  of  the  Eye  .  .  .  .22 

Accommodation  .  .  .  .32 

Convergence       .  .  .  .  .41 


CHAPTER  III 

Methods  of  Determining  the  Refraction  .      53 

Acuteness  of  Yision       .  .  .  .55 

Sclieiner's  Method  .  .  .  .64 


CHAPTER  lY 

The  Ophthalmoscope       .  .  .  .66 

The  Indirect  Method      .  .  .  .66 

The  Direct  Method        .  .  .  .73 


Vlll  CONTENTS 


CHAPTER  Y 

PAGE 

Retinoscopy         .  .  .  .  .82 


CHAPTER  YI 

Hypermetropia  .....    117 
Aphakia    ......    132 


CHAPTER  YII 
Myopia  ......    134 

CHAPTER  YIII 

Astigmatism         .....    156 
Anisometropia    .....    184 

CHAPTER  IX 

Presbyopia  .  -    .  .  .  .    187 

Paralysis  of  the  Accommodation .  .  .     196 

Spasm  of  the  Accommodation      .  .  .197 

CHAPTER  X 
Strabismus  .....    200 


CONTENTS  IX 


CHAPTER  XI 

PAGE 

Asthenopia  .  - .  .  .  .  224 

Accommodative  ....  226 

Muscular  .....  228 

Eetinal  .....  234 


CHAPTER  XII 

Spectacles  .....  237 

Cases      .  .  .  .  .  .  244 

Appendix  .....  259 

Regulations  fok  Army,  Navy,  &c.  .  .  261 

Test  Types  .....  265 


LIST  OF  ILLUSTRATIONS 


No. 

1.  Eeflection  by  a  plane  surface 

2.  Virtual  image  formed  by  a  plane  mirror 

3.  Reflection  by  a  concave  surface 

4.  Ditto  ditto 

5.  Eeflection  by  a  convex  surface 

6.  Eefraction  by  a  plane  surface 

7.  Eefraction  by  a  prism 

8.  Ditto     ditto       . 

9.  Eefraction  by  a  spherical  surface 

10.  Ditto  ditto 

11.  Formation  of  convex  lenses  . 

12.  Different  forms  of  lenses 

13.  Eefraction  of  rays  (secondary  axes)  by  a  convex  lens 

14.  Eefraction  of  parallel  rays  by  a  convex  lens 

15.  Ditto  ditto 

16.  Properties  of  a  biconvex  lens 

17.  Ditto  ditto 

18.  Properties  of  a  biconcave  lens 

19.  Eefraction  of  parallel  rays  by  a  concave  lens 

20.  Formation  of  an  inverted  image 

21.  Eeal  inverted  image  formed  by  a  convex  lens    . 

22.  Virtual  image  formed  by  a  convex  lens 

23.  Virtual  image  formed  by  a  concave  lens 

24.  Diagram  of  eye  showing  the  cardinal  points 

25.  Formation  of  inverted  image  on  the  retina 

26.  Emmetropic,  hypermetropic,  and  myopic  eyeballs 

27.  Eye  represented  by  a  biconvex  lens    . 

28.  Formation  of  visual  angle     . 

29.  Diagram  of  accommodation  . 

30.  Scheiner's  method  of  finding  the  punctum  proximum 

31.  Amovint  of  accommodation  at  different  ages 

32.  Diagram  representing  the  convergence 

33.  Landolt's  ophthalmo-dynamometer 

34.  Diagi-am  of  the  relative  accommodation 

35.  Angle  subtended  at  nodal  point  by  test  type     . 


LIST    OF    ILLUSTEATIONS  XI 

No.  PAGE 

36.  Sclieiner's  method  .... 

37.  Image  formed  in  emmetropia  by  the  indirect  ophthalmo 

seopic  method     .... 

38.  Image  formed  in  hypermetropia 

39.  Image  formed  in  myopia        ... 

40.  Size  of  the  image  in  emmetropia  for  different  distances 

of  the  objective  .... 
41  &  42.  Decrease  of  the  image  in  hypermetropia  on  with 
drawing  the  objective 

43.  Image  formed  in  emmetropia 

44.  Image  formed  in  hypermetropia 

45.  Image  formed  in  myopia 

46.  Direct  ophthalmoscopic  examination  in  emmetropia 

47.  Estimation  of  hypermetropia  by  the  ophthahnoscope 

48.  Estimation  of  myopia  by  the  ophthalmoscope   . 

49.  Kays  coming  from  the  hypermetropic  eye 

50.  Eays  coming  from  the  myopic  eye 

51.  Position  of  light  for  retinoscopy 

52.  Light  with  diaphragm 

53.  Plane  mirror 

54.  Shadows  in  retinoscopy 

55.  Eetinoscopy  with  the  plane  mirror 

56.  Image  formed  in  myopia 

57.  Image  formed  in  hypermetropia 

58.  Oblique  shadows  in  astigmatism 

59.  Cause  of  oblique  shadows 

60.  Band-like  shadows . 

61.  Band-like  shadows . 

62.  Eecording  the  astigmatism  . 

63.  Retinoscopy  with  the  concave  mirror 

64.  Shadows  with  the  concave  mirror 

65.  Movements  of  the  shadow  with  the  concave  mirror 

66.  Refraction  of  a  hypermetropic  eye 

67.  Refraction  increased  by  changes  in  the  lens 

68.  Correction  by  a  biconvex  lens 

69.  Accommodation  at  different  ages  in  hypermetrope  of  3  D 

70.  Refraction  of  a  myopic  eye   . 

71.  Ditto  ditto 

72.  Correction  by  a  biconcave  lens 


Xll  LIST    OF    ILLUSTEATIONS 

No.  PAGE 

73.  Section  of  a  myopic  eyeball  .  .  .  139 

74.  Accommodation  at  different  ages  in  a  myope  of  2  D.       .  140 

75.  Size  of  retinal  image  in  myopia  .  .  .  145 

76.  Section  of  cone  of  light  after  passing  through  an  astig- 

matic cornea.      .  .  .  .  .159 

77.  Diffusion  patches  when  the  cone  is  divided    at   right 

angles  .  ,  „   .  .  .  .  159 

78.  Interval  of  Sturm        ^         .  .  .  .  160 

79.  Simple  hypermetropic  astigmatism    .  .  .  161 

80.  Compound  hypermetropic  astigmatism  .  .  162 

81.  Simple  myopic  astigmatism  ...  162 

82.  Compound  myopic  astigmatism     .      .  .  .  162 

83.  Mixed  astigmatism  .  .  .  .  163 

84.  Astigmatic  clock  face  ....  170 

85.  Astigmatic  fan      .....  171 

86.  Erect  image  of  a  disc  seen  through  an  astigmatic  cornea.  172 

87.  Same  disc  seen  by  the  indirect  method  .  .  172 

88.  Tweedy's  optometer  ....  179 

89.  Diagram  of  the  accommodation  .  .  .188 

90.  Angle  a  in  emmetropia        ....  201 

91.  Angle  a  in  hypermetropia  .  .  ..  .  202 

92.  Angle  a  in  myopia  ....  202 

93.  Diagram  of  primary  and  secondary  deviation  .  .  204 

94.  Strabismometer     .....  206 

95.  Method  of  measuring  the  angle  of  the  strabismus  .  208 

96.  Diagram  representing  convergent  strabismus .  .  210 

97.  Diagram  representing  divergent  strabismus    .  .  216 

98.  Worth's  amblyoscope  ....  220 

99.  Stereoscopic  slide .....  222 

100.  Graefe's  test  for  insufficiency  of  internal  recti  miiscles  .  232 

101.  Scale  for  testing  latent  deviation  at  the  reading  distance.  233 

102.  Convex  and  concave  glasses  acting  as  prisms  .  .  234 

103.  Bifocal  lens  .  .  .  .  .241 

104.  Bifocal  lens  .  .  .  .  .241 

105.  Invisible  bifocal  lens  ....  242 


Lithographic  Plate  opposite  page  147  ; 

1,  2,  and  3.     Drawn  from  myopic  patients. 
4.  Copied  from  Atlas  of  Wecker  and  Jaeger. 
Test  types  ....  168,  265 


THE  REFEACTION  OF  THE  EYE 


CHAPTER  I 

OPTICS 

Light  is  propagated  from  a  luminous  point  in  every 
plane  and  in  every  direction  in  straight  lines ;  these 
lines  of  directions  are  called  rays.  Rays  travel  with 
the  same  rapidity  so  long  as  they  remain  in  the  same 
medium. 

The  denser  the  medium^  the  less  rapidly  does  the 
ray  of  light  pass  through  it. 

Rays  of  light  diverge,  and  the  amount  of  diverg- 
ence is  proportionate  to  the  distance  of  the  point 
from  which  they  come ;  the  nearer  the  source  of  the 
rays,  the  more  they  diverge. 

When  rays  proceed  from  a  distant  point  such  as  the 
sun,  it  is  impossible  to  show  that  they  are  not  parallel  ; 
and  in  dealing  with  rays  which  enter  the  eye,  it  will 
be  sufficiently  accurate  to  assume  them  to  be  parallel 
when  they  proceed  from  a  point  at  a  greater  distance 
than  6  metres. 

A  ray  of  light  meeting  with  a  body  may  be  absorbed, 

1 


2  THE    REPEACTION    OF    THE    EYE 

reflected,  or  if  it  is  able  to  pass  through  this  body  it 
may  be  refracted. 

Reflection 

Reflection  hy  a  Plane  Surface 

Reflection  takes  place  from  any  polished  surface, 
and  according  to  two  laws. 

1st.  The  angle  of  reflection  is  equal  to  the  angle 
of  incidence. 

2nd.  The  reflected  and  incident  rays  are  both  in 
the  same  plane,  which  is  perpendicular  to  the  reflect- 
ing surface. 

Fig.  1. 


Thus,  if  A  B  be  the  ray  incident  at  b,  on  the  mirror 
c  D,  and  B  E  be  the  ray  reflected,  the  perpendicular 
p  B  will  divide  the  angle  a  b  e  into  two  equal  parts, 
the  angle  a  b  p  is  equal  to  the  angle  p  b  e  ;  while 
A  B,  p  B,  and  e  b  lie  in  the  same  plane. 

When  reflection  takes  place  from  a  plane  surface, 
the  image  is  projected  backwards  to  a  distance  behind 
the  mirror  equal  to  the  distance  of  the  object  in  front 
of  it,  the  image  being  of  the  same  size  as  the  object. 

Thus  in  Fig.  2  the  image  of  the  candle  c  is  formed 
behind    the    mirror    m,   at  &,  a  distance  behind  the 


REFLECTION 


mirror  equal  to  the  distance  of  the  candle  in  front  of 
it;  an  observer's  eye  placed  at  e  would  receive  the 
rays  from  c  as  if  they  came  from  c\ 


Fig.  2. 


M.  The  mirror,    c.  The  candle,   c'.  The  virtual  image  of  the  candle. 
E.  The  eye  of  the  observer  receiving  rays  from  the  mirror. 

The  image  of  the  candle  so  formed  by  a  plane  mirror 
is  called  a  virhial  image.  ^ 

Reflection  hy  a  Concave  Surface 
A  concave  surface  may  be  looked  upon  as  made  up 
of  a  number  of  planes  inclined  to  each  other. 

Parallel  rays  falling  on  a  concave  mirror  are  re- 
flected as  convergent  rays^  which  meet  on  the  axis  at 
a  point  (f,  Fig.  3)  called  the  priiicipal  focus,  midway 
between  the  mirror  and  its  optical  centre  c.  The  dis- 
tance of  the  principal  focus  from  the  mirror  is  called 
the  focal  length  of  the  mirror. 


4  THE    REFRACTION    OF   THE    EYE 

If  the  luminous  point  be  situated  at  f,  then  the 
diverging  rays  would  be  reflected  as  parallel  to  each 
other  and  to  the  axis. 

If  the  luminous  point  is  at  the  centre  of  the  con- 
cavity of  the  mirror  (c),  the  rays  return  along  the 
same  lines,  so  that  the  point  is  its  own  image. 

If  the  luminous  point  be  at  a  the  focus  will  be  at  a, 

Fig.  3. 


and  it  is  obvious  that  if  the  luminous  point  be  moved 
to  a,  its  focus  will  be  at  a;  these  two  points  therefore, 
A  and  a,  bear  a  reciprocal  relation  to  each  other,  and 
are  called  conjugate  foci. 

If  the  luminous  point  is  beyond  the  centre,  its  con- 
jugate focus  is  between  the  principal  focus  and  the 
centre. 

If  the  luminous  point  is  betAveen  the  principal  focus 
and  the  centre,  then  its  conjugate  is  beyond  the 
centre ;  so  that  the  nearer  the  luminous  point  ap- 
proaches the  principal  focus,  the  greater  is  the  dis- 
tance at  which  the  reflected  rays  meet. 

If  the  luminous  point  be  nearer  the  mirror  than  the 


EEFLECTION  5 

principal  focus  (f),  the  rays  will  be  reflected  as  diver- 
gent, and  will  therefore  never  meet :  if,  however,  we 
continue  these  diverging  rays  backwards,  they  will 
unite  at  a  point  (h)  behind  the  mirror ;  this  point  is 
called  the  virtual  focus,  and  an  observer  situated  in 

Fig.  4. 


the  path  of  reflected  rays  will  receive  them  as  if  they 
came  from  this  point. 

Thus  it  follows  that — 

Concave  mirrors  produce  two  kinds  of  images  or 
none  at  all,  according  to  the  distance  of  the  object,  as 
may  be  seen  by  looking  at  one^s  self  in  a  concave  mirror. 
If  the  mirror  is  placed  nearer  than  its  principal  focus, 
then  one  sees  an  enlarged  virtual  image,  which  in- 
creases slightly  in  size  as  the  concave  mirror  is  made 
to  recede;  this  image  becomes  confused  and  disap- 
pears as  the  principal  focus  of  the  mirror  is  reached  : 
on  moving  the  mirror  still  farther  away  (that  is  be- 
yond its  focus)  one  obtains  an  enlarged  inverted 
image,  which  diminishes  as  the  mirror  is  still  further 
withdrawn. 


b  THE    REFRACTION    OF    THE    EYE 

Reflection  hy  a  Convex  Surface 
Parallel  rays  falling  on  a  convex  surface  are  reflected 
as  divergent,  hence  never  meet ;  but  if  tlie  diverging 
rays  thus  formed  are  carried  backwards  by  lines, 
then  an  imaginary  image  is  formed  Avhich  is  called 
negative,  and  at  a  point  called  the  principal  focus  (r). 
Foci  of  convex  mirrors  are  therefore  virtual;  and 
the  image,  whatever  the  position  of  the  object,  is 
always  virtual,  erect,  and  smaller  than  the  object. 

Fia.  5. 


The  radius  of  the  mirror  is  double  the  principal 
focus. 

Refraction 

Refraction  hy  a  Plane  Surface 

A  ray  of  light  passing  through  a  transparent  me- 
dium into  another  of  a  different  density  is  refracted, 
unless  the  ray  fall  perpendicular  to  the  surface  sepa- 
rating the  two  media,  when  it  continues  its  course 
without  undergoing  any  refraction   (Fig.   G,   H  k). 


REFRACTION 


A  ray  is  called  incident  before  entering  the  second 
medium,  emergent  after  leaving  it. 

A  ray  passing  from  a  rarer  to  a  denser  medium  is 
refracted  towards  the  perpendicular;  as  shown  in 
Fig.  6,  the  ray  A  b  is  refracted  at  b,  towards  the  per- 
pendicular p  p. 

In  passing  from  the  denser  to  the  rarer  medium 
the  ray  is  refracted  from  the  perpendicular;  b  d  is 
refracted  at  c,  from  p  p  (Fig.  6). 

Fig.  6. 


Keflection  accompanies  refraction,  the  ray  dividing 
itself  at  the  point  of  incidence  into  a  refracted  portion 
B  c,  and  a  reflected  portion  b  e. 

The  amount  of  refraction  is  the  same  for  any 
medium  at  the  same  obliquity,  and  is  called  the  index 
of  refraction;  air  is  taken  as  the  standard,  and  is 
called  1 ;  the  index  of  refraction  of  water  is  1*3,  that 
of  glass  1*5.  The  diamond  has  almost  the  highest 
refractive  power  of  any  transparent  substance,  and 


8  THE    REFRACTION    OP    THE    EYE 

has  an  index  of  refraction  of  2'4.  The  cornea  has  an 
index  of  refraction  of  1"3^  and  the  lens  1*4. 

The  refractive  power  of  a  transparent  substance  is 
not  always  in  proportion  to  its  density. 

If  the  sides  of  the  medium  are  parallel_,  then  all 
rays  except  those  perpendicular  to  the  surface  which 
pass  through  without  altering  their  course  are  re- 
fracted twice,  as  at  b  and  c  (Fig.  6),  and  continue  in 
the  same  direction  after  passing  through  the  medium 
as  they  had  before  entering  it. 

If  the  two  sides  of  the  refracting  medium  are  not 
parallel,  as  in  a  prism,  the  rays  cannot  be  perpen- 
dicular to  more  than  one  surface  at  a  time. 

Therefore  every  ray  falling  on  a  prism  must  un- 
dergo refraction,  and  the  deviation  is  always  towards 
the  base  of  the  prism. 

The  relative  direction  of  the  rays  is  unaltered 
(Fig.  7). 

Fig.  7.  Fia.  8. 


If  D  M  (Fig.  8)  be  a  ray  falling  on  a  prism  (a  b  c)  at 
M,  it  is  bent  towards  the  base  of  the  prism,  assuming 
the  direction  m  n;  on  emergence  it  is  again  bent  at  n; 
an  observer  placed  at  e  would  receive  the  ray  as  if  it 
came  from  k;  the  angle  k  h  d  formed  by  the  two  lines 


REFEACTION  9 

at  H  is  called  the  angle  of  deviation,  and  is  about  half 
the  size  of  the  'principal  angle  formed  at  A  by  the  two 
sides  of  the  prism. 

Refraction  by  a  Spherical  Surface. 

Parallel  rays  passing  through  a  spherical  surface 
separating  media  of  different  density  do  not  continue 
parallel^  but  are  refracted,  so  that  they  meet  at  a 
point  called  the  principal  focus. 

If  parallel  rays  k,  d,  e,  fall  on  A  b,  a  spherical  sur- 
face separating  the  media  m  and  n  of  which  n  is  the 
denser;  ray  d,  which  strikes  the  surface  of  a  b  at  right 
angles,  passes  through  without  refraction,  and  is  called 
the  principal  axis  ;  ray  K  will  strike  the  surface  at  an 
angle,  and  will  therefore  be  refracted  towards  the 
perpendicular  c  J,  meeting  the  ray  d  at  P;  so  also  with 
ray  E,  and  all  rays  parallel  in  medium  m.  The  point 
F  where  these  rays  meet  is  the  principal  focus,  and  the 
Fig.  9. 


distance  between  the  principal  focus  and  the  curved 
surface  is  spoken  of  as  the  principal  focal  distance. 
Rays  proceeding  from  f  will  be  parallel  in  m  after 


10  THE    EP]FRACT10N    OF    THE    EYE 

passing  througli  the  refracting  surface.  Rays  parallel 
in  medium  n  will  focus  at  f',  which  is  called  the  ante- 
rior focus. 

Had  the  rays  in  medium  m  been  more  or  less  diver- 
gent^ they  would  focus  on  the  principal  axis  at  a 
greater  distance  than  the  principal  focus,  say  at  H; 
and  conversely  rays  coming  from  h  would  focus  at  G; 
these  two  points  are  then  conjugate  foci. 

When  the  divergent  rays  focus  at  a  point  on  the 
axis  twice  the  distance  of  the  principal  focus,  then  its 
conjugate  will  be  at  an  equal  distance  on  the  other 
side  of  the  curved  surface. 

If  rays  proceed  from  a  point  o,  nearer  the  surface 
than  its  principal  focus,  they  will  still  be  divergent 
after  passing  through  A  b,  though  less  so  than  before, 
and  will  therefore  never  meet;   by  continuing  these 

Fig.  10. 


rays  backwards  they  will  meet  at  L,  so  that  the  conju- 
gate focus  of  o  will  be  at  l,  on  the  same  side  as  the 
focus;  and  the  conjugate  focus  will  in  this  case  be 
spoken  of  as  negative. 


U 


LENSES  11 

Refraction  hy  Lenses 

Refraction  by  lenses  is  somewhat  more  complicated. 

A  lens  is  an  optical  contrivance  usually  made  of 
glass,  and  consists  of  a  refracting  medium  with  two 
opposite  surfaces,  one  or  both  of  which  may  be  seg- 
ments of  a  sphere;  they  are  then  called  spherical 
lenseSj  of  which  there  are  six  varieties. 

Fig.  11. 


1.  Plano-convex,  the  segment  of  one  sphere  (Fig. 

11,    B). 

2.  Biconvex,  segments  of  two  spheres  (Fig.  11,  a). 

3.  Converging  concavo-convex,  also  called  a  con- 
verging meniscus. 

4.  Plano-concave. 

5.  Biconcave. 

6.  Diverging  concavo-convex,  called  also  a  diverg- 
ing meniscus. 

Lenses  may  be  looked  upon  as  made  up  of  a  number 
of  prisms  with  different  refracting  angles — convex 
lenses,  of  prisms  placed  with  their  bases  together; 
concave  lenses,  of  prisms  with  their  edges  together. 

A  ray  passing  from  a  less  refracting  medium   (as 


12  THE    EEFEACTION    OP    THE    EYE 

air)  througli  a  lens_,  is  deviated  towards  the  thickest 
part,  therefore  the  first  three  lenses,  which  are  thickest 
at  the  centre,  are  called  converging  ;  and  the  others, 
which  are  thickest  at  the  borders,  diverging. 

Fig.  12. 


A  line  passing  through  the  centre  of  the  lens 
(called  the  optical  centre),  at  right  angles  to  the  sur- 
faces of  the  lens,  is  termed  the  principal  axis,  and  any 
ray  passing  through  that  axis  is  not  refracted. 

All  other  rays  undergo  more  or  less  refraction. 

Rays  passing  through  the  optical  centre  of  a  lens, 
but  not  through  the  principal  axis,  suffer  slight  devia- 
tion, but  emerge  in  the  same  direction  as  they  entered. 
These  are  called  secondary  axes  (Fig.  ]3).  The 
deviation  in  thin  lenses  is  so  slight  that  they  are 
usually  assumed  to  pass  through  in  a  straight  line. 

Parallel  rays  falling  on  a  biconvex  lens  are  ren- 
dered convergent ;  thus  in  Fig.  14  the  rays  A,  B,  c, 
strike  the  surface  of  the  lens  (l)  at  the  points  d,  e,  f; 
the  centre  ray  (b)  falls  on  the  lens  at  e  perpendicular 
to  its  surface,  and  therefore  passes  through  in  a 
straight  line ;  it  also  emerges  from  the  lens  at  right 


BICONVEX    LENSES  13 

angles  to  its  opposite  surface^  and  so  continues  its 
course  without  deviation ;  but  the  ray  a  strikes  the 
surface  of  the  lens  obliquely  at  d,  and  as  the  ray  is 
passing  from  one  medium  (air)  to  another  (glass) 
Fig.  13. 


Lens  with  secondaryaxes  undergoing  slight  deviation. 

which  is  of  greater  density,  it  is  bent  towards  the 
perpendicular  of  the  surface  of  the  lens^  shown  by  the 
dotted  line  m  k  ;  the  ray  after  deviation  passes  through 
the  lenSj  striking  its  opposite  surface  obliquely  at  o, 

Fig.  14. 


and  as  it  leaves  the  lens^  enters  the  rarer  medium 
(air),  being  deflected  from  the  perpendicular  n  o ;  it 


14  THE    REFRACTION    OF    THE    EYE 

is  now  directed  to  h^  where  it  meets  the  central  ray 
B  H ;  ray  c^  after  undergoing  similar  refractions,  meets 
the  other  rays  at  h,  and  so  also  all  parallel  rays  falling 
on  the  biconvex  lens  (l). 

Parallel   rays,  therefore,  passing  through  a  convex 
lens  (l)  are  brought  to  a  focus  at  a  certain  fixed  point 

Fig.  15. 


(a)  beyond  the  lens;  this  point  is  the  ^principal  focus, 
and  the  distance  of  this  focus  from  the  lens  is  called 
the  focal  length  of  the  lens. 

Rays  from  a  luminous  point  placed  at  the  principal 
focus  (a)  emerge  as  parallel  after  passing  through  the 
lens. 

Divergent  rays  from  a  point  (b)  outside  the  princi- 
pal focus  (f,  Fig.  16)  meet  at  a  distance  beyond  (f') 
the  principal  focus  on  the  other  side  of  the  lens  (l), 
and  if  the  distance  of  the  luminous  point  (b)  is  equal 
to  twice  the  focal  length  of  the  lens,  the  rays  will  focus 
at  a  point  (c)  the  same  distance  on  the  opposite  side 
of  the  lens  ;  rays  coming  from  c  would  also  focus  at 
B  :  they  are  therefore  called  conjugate  foci,  for  we 
can  indifferently  replace  the  image  (c)  by  the  object 
(b),  and  the  object  (b)  by  the  image  (c). 


BICONVEX    LENSES  15 

If  the  luminous  point  (d)  be  between  the  lens  and 
the  principal  focus  (p),  then  the  rays  will  issue  from 
the  lens  divergent^  though  less  so  than  before  enter- 
ing it ;  and  if  we  prolong  them  backwards  they  will 

Fig.  16. 


meet  at  a  point  (h)  further  from  the  lens  than  the 
point  D ;  H  will  therefore  be  the  virtual  focus  of  d,  and 
the  conjugate  focus  of  d  may  be  spoken  of  as  negative. 

Biconvex  lenses  have  therefore  two  principal  foci, 
r  and  f'^  one  on  either  side^  at  an  equal  distance  from 
the  centre. 

In  ordinary  lenses^  and  those  in  which  the  radii  of 
the  two  surfaces  are  nearly  equal^  the  principal  focus 
closely  coincides  with  the  centre  of  curvature. 

We  have  assumed  the  luminous  point  to  be  situated 
on  the  principal  axis;  supposing,  however,  it  be  to  one 
side  of  it  as  at  e  (Fig.  17),  then  the  line  (e  f)  passing 
through  the  optical  centre  (c)  of  the  lens  (l)  is  a 
secondary  axis,  and  the  focus  of  the  point  e  will  be 
found  somewhere  on  this  line,  say  at  f,  so  that  what 
has  been  said  respecting  the  focus  of  a  luminous  point 
on  the  principal  axis  (a  b)  is  equally  true  for  points 
on  a  secondary  axis,  provided  ahvays  that  the  inclina- 


16  THE    REFRACTION    OF    THE    EYE 

tion  of  this  secondary  axis  is  not  too  great,  when 
the  focus  will  become  imperfect  on  account  of  the 
spherical  aberration  which  will  be  produced. 

Fig.  17. 


In  biconcave  lenses  the  foci  are  always  virtual, 
whatever  the  distance  of  the  object. 

Rays  of  light  parallel  to  the  axis  diverge  after 
refraction,  and  if  their  direction  be  continued  back- 
ward they  will  meet  at  a  point  termed  the  principal 
focus  (Fig.  18,  f). 

Fig.  18. 


Fig.  19  shows  the  refraction  of  parallel  rays  by  a 
biconcave  lens  (l)  ;  the  centre  ray  B  strikes  the  lens 
at  E  perpendicular  to  its  surface,  passing  through 
without  refraction,  and  as  it  emerges  from  the  oppo- 
site side  of  the  lens  perpendicular  to  its  surface,  it 


FORMATION    OF    IMAGES  17 

continues  in  a  straight  line;  the  ray  A  strikes  the  lens 
obliquely  at  d  and  is  refracted  towards  the  perpen- 
dicular, shown  by  the  dotted  line  G  h  ;  the  ray  after 

Fig.  19. 


deviation  passes  through  the  lens  to  k,  where,  on 
entering  the  medium  of  less  density  obliquely,  it  is 
refracted  from  the  perpendicular  o  p,  in  the  direction 
K  M  ;  the  same  takes  place  witli  ray  c  at  f  and  n  ;  so 
also  with  all  intermediate  parallel  rays. 

Formation  of  Images. — To  illustrate  the  formation 
of  images  the  following  simple  experiment  may  be 
carried  out:  —  Take  a  screen  with  a  small  perforation 
and  place  on  one  side  of  it  a  candle,  and  on  the 
other  side  a  sheet  of  white  cardboard  at  a  suitable 
distance  to  receive  any  image :  rays  diverge  from 
the  candle  in  all  directions,  most  of  those  falling 
on  the  screen  are  intercepted  by  it;  but  some  few 
pass  through  the  perforation  and  form  an  image  of 
the  candle  on  the  cardboard,  the  image  being  in- 
verted because  the  rays  cross  each  other  at  the 
orifice.  It  can  further  be  shown  that  when  the 
candle  and  cardboard  are   equally  distant  from  the 

2 


18  THE    REFRACTION    OF    THE    EYE 

perforated  screen,  the  candle  flame  and  its  image  will 
be  of  the  same  size.  If  the  cardboard  be  moved 
further  from  the  screen  the  image  is  enlarged,  if 
it  be  moved  nearer  it  is   diminished;  if  we  make  a 

Fig.  20. 


dozen  perforations  in  the  screen,  a  dozen  images 
will  be  found  on  the  cardboard,  if  a  hundred  then  a 
hundred;  but  if  the  apertures  are  so  close  together 
that  the  images  overlap,  then  instead  of  so  many 
distinct  images  we  get  a  general  illumination  of  the 
cardboard. 

The  image  of  an  object  is  the  collection  of  the  foci 
of  its  several  points ;  the  images  formed  by  lenses 
are,  as  in  the  case  of  the  foci,  real  or  virtual.  Images 
formed,  therefore,  by  convex  lenses  may  be  real  or 
virtual. 

In  Fig.  21,  let  A  b  be  a  candle  situated  at  an 
infinite  distance ;  from  the  extremities  of  A  b  draw 
two  lines  passing  through  the  optical  centre  (c)  of  a 
biconvex  lens,  then  the  image  of  a  will  be  formed 
somewhere  on  the  line  AC  a  (termed  a  secondary  axis), 
say  at  a;  the  image  of  b  will  be  formed  on  the  line 
B  c  6,  say  at  h;  therefore  6  a  is  a  small  inverted  image 


FOEMATION    OF    IMAGES  19 

of  the  candle  a  b,  formed  at  the  principal  focus  of  the 

convex  lens.     Had  the  candle  been  placed  at  twice 

the  focal  distance  of  the  lens,  then  its  inverted  image 

Fig.  21. 


Eeal  inverted  image  formed  by  convex  lens. 

would  be  formed  at  a  corresponding  point  on  the 
opposite  side  of  the  lens,  and  would  be  of  the  same 
size  as  the  object. 

If  the  candle  be  at  the  principal  focus  (f),  then  the 
image  is  at  an  infinite  distance,  the  rays  after  refrac- 
tion being  parallel. 

If,  however,  the  candle  (a  b)  be  nearer  the  lens  than 
Fig.  22. 


Virtual  image  formed  by  convex  lens. 

the  focus,  then  the  rays  which  diverge  from  the  candle 
will,   after  passing  through  the  convex  lens,  be  still 


20  THE    REFRACTION    OF    THE    EYE 

divergent,  so  that  no  image  is  formed ;  an  eye  placed 
at  E  would  receive  the  rays  from  a  b  as  if  they  came 
from  ah;  a  h  is  therefore  a  virtual  image  of  A  b,  erect 
and  larger  than  the  object,  and  formed  on  the  same 
side  of  the  lens  as  the  object. 

Images  formed   by    biconcave    lenses   are    always 
virtual,  erect,  and  smaller  than  the  object.     Let  A  b 

Fig.  23. 


Virtual  image  formed  by  concave  lens. 

be  a  candle,  and  f  the  principal  focus  of  a  biconcave 
lens ;  draw  from  a  and  b  two  lines  through  c,  the 
optical  centre  of  the  lens,  and  lines  also  from  a  and  b 
parallel  to  the  axis ;  after  passing  through  the  lens 
they  diverge  and  have  the  appearance  of  coming  from 
a  h,  which  is  therefore  the  virtual  image  of  A  b. 

A  real  image  can  be  projected  on  to  a  screen,  but  a 
virtual  one  can  only  be  seen  by  looking  through  the 
lens. 


EE  FRACTION  21 

CHAPTER  II 

REFRACTION.       ACCOMMODATION.       CONVERGENCE 

The  eye  may  be  looked  upon  as  an  optical  instru- 
ment, a  sort  of  photographic  camera,  designed  to  pro- 
duce by  means  of  its  refracting  system  a  small  and 
inverted  picture  of  surrounding  objects  upon  the 
retina;  the  stimulation  produced  by  this  picture  on 
the  retina  is  conveyed  through  the  optic  nerve  to  the 
ganglion  cells  of  the  cortex,  that  part  of  the  brain 
known  as  the  optical  area ;  this  excitation  of  the  gan- 
glion cells  of  this  centre  becomes  sensation,  and  thus 
it  is  that  the  retinal  picture  comes  within  the  domain 
of  consciousness,  and  the  brain  interprets  the  im- 
pressions transmitted  to  it  from  the  retina. 

Immediately  behind  the  transparent  retina  is  a  layer 
of  pigment,  which  absorbs  some  of  the  rays  of  light  as 
soon  as  the  image  is  formed ;  were  this  not  so  the  rays 
would  be  reflected  to  other  parts  of  the  retina,  and 
cause  much  dazzling,  considerably  interfering  with 
vision;  this  is  the  case  in  those  persons  who  have  a 
congenital  absence  of  pigment,  and  who  are  known  as 
albinos. 

The  refracting  system  of  the  eye  is  so  arranged, 
that  very  little  spherical  or  chromatic  aberration 
takes  place,  as  is  the  case  with  ordinary  optical 
instruments. 

For  distinct  and  accurate  binocular  vision  the  fol- 
lowing conditions  are  necessary : 


22  THE    EEPRACTION    OF    THE    EYE 

1.  That  a  well-defined  inverted  image  be  formed 
on  tlie  layer  of  rods  and  cones  at  tlie  yellow  spot  of 
each  eye. 

2.  That  the  impression  here  received  be  conveyed 
to  the  brain. 

In  a  work  of  this  character  the  first  of  these  condi- 
tions alone  concerns  us^  and  for  the  carrying  out  of 
this — the  media  being  transparent — three  important 
factors  call  for  a  separate  description,  viz.  : 

Refraction. 

Accommodation. 

Convergence. 

Refraction. — This  term  is  used  to  express  the  optical 
condition  of  the  eye  in  a  state  of  rest.  There  are  three 
refracting  surfaces  in  the  eye — the  anterior  surface  of 
the  cornea,  the  anterior  surface  of  the  lens,  and  the 
anterior  surface  of  the  vitreous ;  and  three  refracting* 
media — the  aqueous,  the  lens,  and  the  vitreous. 
These  together  make  up  the  dioptric  system,  and 
may  for  the  sake  of  simplicity  be  looked  upon  as 
equal  to  a  convex  lens  of  about  23  mm.  focus.  What 
was  said  about  convex  lenses  applies  equally  to  the 
eye  as  an  optical  instrument. 

A  ray  of  light  falling  on  the  cornea  does  not,  how- 
ever, follow  the  simple  direction  we  might  imagine 
when  considering  the  eye  merely  as  a  lens  of  23  mm. 
focus  :  the  eye  must  be  looked  upon  as  a  compound 
refracting  system,  composed  of  a  spherical  surface  and 
a  biconvex  lens.  To  enable  us  to  understand  the 
course  of  a  ray  of  light  through  the  eye,  it  is  neces- 


EEFEACTION 


23 


sary  to  be  acquainted  with  tlie  cardinal  points  of  this 
compound  system.  Too  much  space  would  be  occu- 
pied to  explain  how  the  position  of  these  points  is 
arrived  at^  but  it  suffices  to  say  that,  having  first 
found  the  cardinal  points  of  the  cornea  and  then  those 
of  the  lens,  the  cardinal  points  of  the  eye  will  be  the 
result  of  these  two  systems  together. 

The  cardinal  points  of  the  eye  are  six  in  number, 
two  prmcipal  points,  two  nodal  points,  and  two  prin- 
cipal foci. 

Fig.  24. 


In  the  above  diagram  of  the  emmetropic  eye  the 
cardinal  points  of  this  compound  system  are  shown, 
all  situated  on  the  optic  axis  (f  a)  :  at  b  are  two  prin- 
cipal points  situated  so  closely  together  in  the  anterior 
chamber  that  they  may  conveniently  be  looked  upon 
as  one  point;  at  N  are  two  nodal  points,  also  close 
together, — for  simplicity  we  shall  consider  them  as 
one  point ;  at  f  is  the  first  principal  focus,  at  a  the 
second  principal  focus.    We  then  have  the  following: 


24  THE    REFRACTION    OF    THE    EYE 

c,  the  cornea ;  l,  the  lens ;  m,  the  macula ;  o,  the  optic 
nerve;  f  a,  the  optic  axis;  b,  the  principal  point;  n, 
the  nodal  point ;  h^  the  centre  of  rotation  of  the  eye, 
9*8  mm.  in  front  of  the  retina;  a,  the  second  principal 
focus  ;  and  F,  the  first  principal  focus. 

The  nodal  points  correspond  nearly  to  the  optical 
centre  of  the  refracting  system,  the  axis  ray  passing 
through  these  points  is  not  refracted  ;  every  ray 
directed  to  the  first  nodal  point  appears  after  refrac- 
tion to  come  from  the  second  point,  and  continues  in 
the  same  direction  to  that  which  it  first  had  :  the 
nodal  points  in  the  eye  are  situated  about  7  mm. 
behind  the  cornea  (Fig.  24,  n). 

The  princijml  points. — When  an  incident  ray  passes 
through  the  first  principal  point,  the  corresponding 
emergent  ray  passes  through  the  second  principal 
point,  but  the  incident  and  emergent  rays  are  not 
parallel;  the  principal  points  are  situated  about  2  mm. 
behind  the  cornea  (Fig.  24,  b). 

The  first  principal  focus  is  that  point  on  the  axis 
Avhere  rays  parallel  in  the  vitreous  meet ;  this  point  is 
about  13-7  mm.  in  front  of  the  cornea  (Fig.  24,  f). 

A  vertical  line  passing  through  this  point  is  called 
the  first  principal  plane. 

The  second  principal  focus  is  that  point  on  the  axis 
where  parallel  rays  meet  after  passing  through  the 
eye,  22*8  mm.  behind  the  cornea  (Fig.  24,  a). 

A  vertical  line  passing  through  this  point  is  called 
the  second  principal  plane. 

A  luminous  point  placed  above  the  principal  axis 
has  its  image  formed  on  the  retina  below  this  axis;  and 


EEFEACTION  25 

inversely^  the  image  of  a  point  below  the  principal 
axis  will  be  formed  above  it.  If  we  replace  these 
two  points  by  an  object  the  same  thing  occurs,  and  we 
get  an  inverted  image  (Fig.  25)  :  it  is  essential  that 
the  method  of  formation  of  these  inverted  images  be 
thoroughly  understood. 

From  every  point  of  an  object  abc  proceed  diverg- 
ent rays.  Some  of  those  rays  coming  from  A,  pass 
through  the  pupil,  and  being  refracted  by  the  dioptric 
system,  come  to  a  focus  on  the  retina  at  a;  some 
coming  from  b,  focus  at  h,  and  some  from  c  at  c.     In 

Fig.  25. 


the  same  way  rays  come  from  every  part  of  the  object 
as  divergent  rays,  and  are  brought  to  a  focus  on  the 
retina ;  so  that  the  retina,  being  exactly  at  the  focal 
distance  of  the  refracting  system,  receives  a  well- 
defined  inverted  image. 

Much  has  been  said  and  written  as  to  why  images 
which  are  formed  in  an  inverted  position  on  the  retina 
should  be  seen  upright,  and  all  sorts  of  ingenious 
explanations  have  from  time  to  time  been  given.  The 
whole  thing  is  entirely  a  matter  of  education  and  ex- 
perience, which  is  supplemented  and  corroborated  by 


26  THE    REFRACTION    OP    THE    EYE 

the  sense  of  touch.  We  have  no  direct  cognizance  of 
the  image  on  the  retina,  nor  of  the  position  of  its 
different  parts,  but  only  of  the  stimulation  of  the 
retina  produced  by  the  image ;  this  stimulation  is 
conducted  by  the  optic  nerve  to  the  brain,  producing 
there  certain  molecular  changes.  We  do  not  actually 
see  the  retinal  image,  but  the  eye  receives  the  rays 
emanating  from  the  object  looked  at,  and  we  refer  the 
sensation  in  the  direction  of  these  rays;  thus,  if  an 
image  is  formed  on  the  upper  part  of  our  retina,  we 
refer  the  sensation  downwards  from  which  the  rays 
must  have  come. 

The  great  advantage  of  inverted  images  is,  that  for 
a  given-sized  pupil  a  much  larger  retinal  picture  can 
be  formed  than  would  be  the  case  if  no  inversion 
took  place;  for  in  the  latter  case  all  images  must 
necessarily  occupy  a  smaller  space  on  the  retina  than 
the  size  of  the  pupil. 

The  refraction  of  the  eye  is  said  to  be  normal  when 
parallel  rays  are  united  exactly  on  the  layer  of  rods 
and  cones  of  the  retina;  in  other  words,  when  the 
retina  is  situated  exactly  at  the  second  principal  focus 
of  the  eye.  This  condition  is  called  emmetropia  {Ifi, 
within;  juirpov,  measure;  w^,  the  eye  (Fig.  26,  a). 
If  parallel  rays  are  focussed  behind  or  in  front  of  the 
retina,  then  the  term  ametropia  (a,  priv. ;  jucrpov, 
measure ;  wxp,  the  eye)  is  used,  and  of  this  there  are 
two  opposite  varieties  : 

Hypermetropia,  when  the  eyeball  is  so  short  that 
parallel  rays  are  brought  to  a  focus  behind  the  retina 
(Fig.  26,  B). 


REFRACTION  27 

Myopia,  when  the  eyeball  is  too  long,  so  that  parallel 
rays  focus  in  front  of  the  retina  (Fig.  26,  c) . 

Fig.  26. 


A.  Emmetropic  eye.     b.  Hypermetropic  eye.     c.  Myopic  eye. 

Emmetropia  in  a  strict  mathematical  sense  is  very 
rare. 

If  we  represent  the  eye  by  a  biconvex  lens,  and  the 
retina  by  a  screen ;  then  it  will  correspond  to  emme- 
tropia when  the  screen  is  situated  at  the  principal 
focus  of  the  lens,  as  e.  Fig.  27 ;  we  represent  hyper- 
metropia  (h)  by  bringing  forward  the  screen,  and 
myopia  (m)  by  moving  it  further  away  from  the 
lens. 

In   all   eyes,    vision   ranges  from  the  far  point  or 


28  THE    EEFRACTION    OF    THE    EYE 

punctum  remotum  (which  in  the  emmetropic  eye  is  at 
infinity)  to  the  near  point  or  punctum  proximum. 

Fig.  27. 


i 


Convex  lens  of  23  mm.  focus.  Parallel  rays  focus  at  e 
(emmetropia)  on  the  screen,  forming  a  well-defined  image 
of  the  object  from  which  rays  come  ;  at  h  (hypermetropia) 
they  form  a  diffusion  patch  instead  of  an  image,  m  (my- 
opia), also  a  diffusion  patch,  the  rays  having  crossed  and 
become  divergent. 

The  near  point  varies  in  the  normal  eye  according 
to  the  amount  of  the  accommodation,  receding  gradu- 
ally as  age  advances;  when  it  has  receded  beyond 
22  cm.  (which  usually  occurs  in  the  emmetropic  eye 
about  the  age  of  forty-five)  the  condition  is  spoken  of 
as  presbyopia. 

Infinity  is  any  distance  beyond  six  metres,  the  rays 
coming  from  a  point  at  or  beyond  that  distance  being 
parallel  or  almost  so. 

The  emmetropic  eye,  therefore,  has  its  far  point,  or 
punctum  remotum,  situated  at  infinity ;  the  hyperme- 
tropic eye  has  its  punctum  remotum  beyond  infinity, 
and  the  myopic  eye  has  its  punctum  remotum  at  a 
finite  distance. 

Generally  the  two  eyes  are  similar  in  their  refrac- 


VISUAL    ANGLE  29 

tion,  though  sometimes  there  is  a  very  great  difference. 
One  eye  may  be  hypermetropic,  the  other  myopic; 
or  one  emmetropic,  the  other  ametropic.  Anisome- 
tropia is  the  term  used  when  the  two  eyes  thus  vary 
in  their  refraction. 

There  may  be  differences  also  in  the  refraction  of 
the  different  meridians  of  the  same  eye — astigmatism. 

In  all  forms  of  ametropia  the  acuteness  of  vision  is 
liable  to  be  diminished.  The  visual  acuteness  usually 
decreases  slightly  as  age  advances,  without  any  dis- 
ease. 

The  visual  acuteness  refers  always  to  central  vision. 
The  yellow  spot  is  the  most  sensitive  part  of  the  retina, 
and  the  sensibility  diminishes  rapidly  towards  the 
periphery.  The  acuteness  is  measured  by  the  size  of 
the  visual  angle,  that  is  the  angle  formed  at  the  pos- 
terior nodal  point,  which  point  closely  coincides  with 
the  posterior  surface  of  the  lens,  and  is  about  15  mm. 
in  front  of  the  yellow  spot. 

In  Fig.  28,  let  c  d  be  an  object  for  which  the  eye  is 
Fig.  28. 


accommodated.  The  lines  c  c,  a  d,  drawn  from  the 
extremities  of  the  object,  cross  at  the  nodal  point  n. 
The  angle  c  n  d  will  be  the  visual  angle  under  which 


30  THE    REFEACTION    OF    THE    EYE 

the  object  c  d  is  seen.  The  size  of  the  angle  depends 
upon  the  distance  of  the  object  as  well  as  upon  its 
magnitude,  and  the  size  of  the  image  thus  formed  on 
the  retina  will  also  depend  upon  the  antero-posterior 
diameter  of  the  eyeball. 

Thus  an  object  a  b,  which  is  as  large  as  c  d,  but 
nearer  to  the  eye,  will  be  seen  under  a  larger  angle, 
the  angle  an  b  being  greater  than  the  angle  c  N  d.  It 
is  also  clear  that  the  image  formed  on  the  retina  will 
be  smaller  at  1,  when  the  antero-posterior  diameter  of 
the  eye  is  less,  as  in  hypermetropia,  than  it  is  at  2  in 
emmetropia,  ^nd  that  it  will  be  larger  in  myopia,  as 
at  3,  where  the  eyeball  is  elongated.  It  is,  therefore, 
easy  to  understand  that  a  patient  may  be  able  to  read 
the  smallest  type  and  still  have  some  defect  of  refrac- 
tion, unless  the  type  be  read  at  its  proper  distance 
(see  Fig.  35). 

It  is  by  the  unconscious  comparison  of  things 
of  known  size,  and  the  amount  of  accommodation 
brought  into  play,  that  we  are  able  to  estimate  the 
distance  of  objects,  and  not  by  the  visual  angle  alone. 

Objects  must  therefore  be  of  a  certain  size,  and  it 
has  been  proved  that  to  enable  us  to  see  a  complex 
figure  like  a  letter  distinctly,  each  part  of  the  figure 
must  be  separated  from  the  other  parts  by  an  interval 
equal  to  not  less  than  the  arc  subtending  an  angle  of 
r  at  the  nodal  point. 

It  has  been  shown  (Fig.  26,  b)  that  in  the  hyper- 
metropic eye  in  a  state  of  rest,  parallel  rays  are 
brought  to  a  focus  behind  the  retina,  so  that  instead 
of  a  clear,  well-defined  image,  we  get  a  circle  of  dif- 


LENSES  31 

fusion.  Convex  glasses  render  parallel  rays  passing 
through  them  convergent,  so  that  by  placing  a  lens 
of  the  right  strength  in  front  of  the  hypermetropic 
eye,  we  bring  forward  its  focus  on  to  the  retiua. 

In  myopia  (Fig.  26,  c)  the  focus  for  parallel  rays 
is  in  front  of  the  retina;  concave  glasses  render 
parallel  rays  passing  through  them  divergent,  so  that 
the  proper  concave  glass  will  carry  back  the  focus  on 
to  the  retina. 

Lenses. — The  lenses  used  for  estimating  the  visual 
acuteness  consist  of  two  kinds,  spherical  and  cylin- 
drical. Spherical  lenses  were  until  recently  numbered 
according  to  their  radii  of  curvature,  which  was 
considered  as  equal  to  their  focal  length  in  inches, 
a" glass  of  1-inch  focus  being  taken  as  a  standard.  To 
this  plan  there  were  several  objections.  The  standard 
glass  being  a  strong  one,  weaker  glasses  had  to  be  ex- 
pressed in  fractions.  Thus  a  glass  of  4-inch  focus  was 
one  fourth  the  strength  of  the  standard  1  inch,  and  was 
expressed  as  J.  In  addition  to  the  trouble  and  incon- 
venience of  working  with  fractions,  the  intervals 
between  the  lenses  were  most  irregular,  and,  moreover, 
the  inches  of  different  countries  vary.  At  the  Oph- 
thalmological  Congress  in  1872  it  was  decided  to  adopt 
a  metrical  scale  of  measurement.  A  lens  of  1-metre 
focus  is  taken  as  the  unit,  and  is  called  a  dioptre;  a  weak 
instead  of  a  strong  glass  therefore  becoming  the  unit, 
a  lens  of  two  dioptres  is  twice  the  strength  of  the 
former,  and  has  a  focal  length  of  half  a  metre.  Thus 
each  lens  is  numbered  according  to  its  refracting 
power,  and  not,  as  in  the  old  system,  according  to  its 


32  THE    KEFEACTION    0¥    THE    EYE 

focal  length;  so  that  we  have  a  series  composed  of 
equidistant  terms.  The  numbers  1  to  20  indicate  the 
uniformly  increasing  power  of  the  glasses. 

The  focal  length  of  a  lens  is  not  expressed  in  the 
dioptric  measurement;  we  have  only  to  remember  that 
it  is  the  inverse  of  the  refracting  power^  so  that  by 
dividing  100  cm.  by  the  number  of  the  lens  we  obtain 
its  focal  length  in  centimetres :  for  example^  if  the 
strength  of  the  lens  be  2  D._,  then  the  focal  length  will 
be  50  cm. ;  if  10  D.,  then  10  cm. 

The  intervals  between  dioptres  is  somewhat  large, 
so  that  decimals,  "25,  '50,  "75  of  a  dioptre,  are  intro- 
duced ;  these  work  easily. 

On  looking  through  a  convex  glass  objects  look 
larger,  through  a  concave  glass  they  look  smaller. 

The  cylindrical  lens  still  remains  to  be  mentioned; 
it  consists  of  a  lens  one  surface  of  which  is  usually 
plane  while  the  other  is  the  segment  of  a  cylinder, 
and  may  be  either  convex  or  concave  :  if  a  convex 
cylinder  be  held  vertically,  the  vertical  meridian  will 
be  plane,  exercising  no  influence  on  rays  passing 
through  it  in  that  meridian;  while  the  horizontal 
meridian  will  be  convex,  and  will  act  as  such  on  rays 
passing  through  it.  The  axis  of  the  cylinders  is 
usually  indicated  by  a  portion  of  the  lens  on  each 
side  being  ground  parallel  to  its  axis. 

Accommodation. — In  the  normal  eye,  in  a  condition 
of  complete  repose,  parallel  rays  come  to  a  focus 
exactly  on  the  rods  and  cones  of  the  retina,  and  the 
object  from  which  the  rays  comes  is  therefore  seen 
distinctly. 


ACCOMMODATION  33 

Eays  from  a  near  object  proceed  in  a  divergent 
direction^  and  come  to  a  focus  behind  the  retina ;  the 
object  would  not  then  be  clearly  seen  unless  the  eye 
possessed  within  itself  the  power  of  bringing  rays 
which  are  more  or  less  divergent  into  union  on  the 
retina. 

This  power  of  altering  the  focus  of  the  eye  is  called 
accommodation,  and  is  due  to  an  alteration  in  the  form 
of  the  lens.  That  the  eye  possesses  this  power  can 
easily  be  proved  in  many  ways,  apart  from  the  con- 
scious muscular  eifort ;  perhaps  as  simple  a  way  as 
any  to  demonstrate  it  to  one's  self  is  to  look  through 
a  net  held  a  short  distance  off  at  some  distant  object. 
Either  the  net  or  the  object  can  be  seen  distinctly, 
but  not  both  at  once.  If  the  meshes  of  the  net  be 
looked  at,  then  the  distant  object  becomes  indistinct, 
and  on  looking  at  the  object  the  meshes  become  con- 
fused. 

Accommodation,  therefore,  increases  the  refraction 
of  the  eye,  and  adapts  it  to  near  objects.  The  changes 
which  take  place  in  the  lens  during  accommodation 
are: 

1st.  The  anterior  surface  becomes  more  convex  and 
approaches  the  cornea. 

2nd.  The  posterior  surface  becomes  slightly  more 
convex,  but  remains  the  same  distance  from  the  cornea. 
That  these  changes  take  place  may  be  proved  in  the 
following  manner: — A  lighted  candle,  or  other  con- 
venient object,  being  held  on  one  side  of  the  eye,  so 
as  to  form  an  angle  of  30°  with  its  visual  axis,  an 
observer  looking  into  the  eye  from  a  corresponding 

3 


34  THE    REFRACTION    OP    THE    EYE 

position  on  the  other  side,  will  see  three  images  of 
the  flame :  the  first  upright,  formed  by  the  cornea ; 
the  second  larger,  upright,  and  formed  by  the  anterior 
surface  of  the  lens;  the  third  smaller  and  inverted, 
formed  by  the  posterior  surface  of  the  lens.  When  the 
accommodation  is  put  in  force,  images  one  and  three 
remain  unchanged  in  shape  and  position ;  image  two, 
which  is  that  formed  by  the  anterior  surface  of  the 
lens,  becomes  smaller,  more  distinct,  and  approaches 
image  one,  proving  that  this  surface  of  the  lens  has 
become  more  convex  and  has  approached  the  cornea. 
In  an  emmetropic  eye  adapted  for  infinity,  it  has  been 
proved  that  the  radius  of  curvature  of  the  anterior 
surface  of  the  lens  is  10  mm. ;  when  accommodated 
for  an  object  13'5  cm.  off  it  is  changed  to  6  mm. 

During  accommodation  the  pupil  becomes  smaller, 
the  central  part  of  the  iris  advances,  while  the  peri- 
pheral part  slightly  recedes. 

The  alteration  in  the  shape  of  the  lens  is  due  to 
the  contraction  of  the  ciliary  muscle,  which  draws 
forward  the  choroid,  thereby  relaxing  the  suspensory 
ligament,  and  allowing  the  elasticity  of  the  lens  to 
come  into  play.  This  elasticity  is  due  to  the  peculiar 
watch-spring  arrangement  of  the  lens  fibres. 

When  the  ciliary  muscle  is  relaxed,  the  suspensory 
ligament  returns  to  its  former  state  of  tension,  and  so 
tightens  the  anterior  part  of  the  capsule,  flattening 
the  front  surface  of  the  lens."^ 

*  Another  theory  of  accommodation  is  Tscherning's,  whose  ex- 
periments have  led  him  to  believe  that  when  the  ciliary  muscle 
contracts  it  increases  the  tension  of  the  zonula,  and  alters  the 
lens  surface  from  a  spherical  to  a  hyperboloid  form. 


PUNCTUM    PROXIMUM  35 

When  the  muscle  is  relaxed  to  its  uttermost,  the 
lens  has  assumed  its  least  convexity,  and  the  eye  is 
then  adapted  for  its  far  point  {punctum  remotum)  (r). 

In  this  condition  the  eye  is  spoken  of  as  being  in  a 
state  of  complete  repose. 

In  the  emmetropic  eye  the  punctum  remotum  is 
situated  at  infinity. 

Fig.  29. 


Diagram  of  lens,  cornea,  &c.  The  right  half  is  represented 
as  in  a  state  of  accommodation,  the  left  half  at  rest. 
A,  The  anterior  chamber,  c.  The  cornea,  l.  The  lens. 
V.  The  vitreous  humour,     i.  The  iris.     m.  Ciliary  muscle. 

When  the  ciliary  muscle  has  contracted  as  much  as 
it  can,  the  lens  has  assumed  its  greatest  convexity, 
and  its  maximum  amount  of  accommodation  is  in 
force.  The  eye  is  now  adapted  for  the  nearest  point 
which  can  be  seen  distinctly;  this  is  called  the  punctum 
proximum  (p). 

The  position  of  the  punctum  proximum  can  be 
determined  in  several  ways ;  the  ordinary  plan  is  to 
place  in  the  patient's  hand  the  small  test  type,  and 
note  the  shortest  distance  at  which  he  can  read  No.  1 
with  each  eye  separately.  Or  we  may  measure  its  posi- 
tion with  the  wire  optometer,  which  consists  of  a  steel 


36  THE    REFRACTION    OF    THE    EYE 

frame  crossed  by  tliin  vertical  wires;  this  is  supported 
in  a  liandle  to  which  a  tape  measure  is  attached;  the 
tape  is  placed  against  the  temple,  and  held  there  while 
the  frame  is  made  gradually  to  recede  from  the 
patient's  eye  we  are  examining,  stopping  as  soon  as 
the  wires  become  distinct,  and  reading  off  the  number 
of  centimetres  on  the  measure. 

Another  excellent  plan  by  which  to  find  the  position 
of  the  punctum  proximum  is  that  of  Scheiner :  close 
in  front  of  the  eye  we  wish  to  examine  is  placed  a 
card  pierced  with  two  small  pinholes,  which  must  not 
be  further  apart  than  the  diameter  of  the  pupil; 
through  these  two  holes  the  patient  is  directed  to 
look  at  a  pin  held  about  one  metre  away  (the  other 
eye  is  of  course  excluded  from  vision  during  the  ex- 
periment);  the  pin  will  be  clearly  and  distinctly  seen. 
We  then  gradually  approach  it  to  the  eye  :  at  a 
certain  place  it  Avill  become  double  :  the  point  at 
which  the  pin  ceases  to  appear  single  will  be  the 
punctum  proximum. 

In  Fig.  30  the  biconvex  lens  L  represents  the  eye. 
Fig.  30. 


D  the  perforated  card,  p  the  pin,  e  e'  the  two  sets  of 
rays  from  p,  which  focus  exactly  at  b,  the  retina.     If, 


AMPLITUDE    OF   ACCOMMODATION 


37 


however,  the  pin  be  brought  nearer,  so  that  the  accom- 
modation is  unable  to  focus  the  two  sets  of  rays,  they 
will  form,  instead  of  one,  two  images  of  the  pin  on  the 
retina  as  at  a.  These  will  be  projected  outwards  as 
crossed  images. 

The  space  between  the  punctum  remotum  and  the 
punctum  proximum  is  called  the  range  of  accommoda- 
tion. 

The  force  necessary  to  change  the  eye  from  its 
punctum  remotum  to  its  punctum  proximum  is  styled 
the  amplitude  of  accommodation.  The  amplitude  of 
accommodation,  therefore,  is  equal  to  the  diiference- 
between  the  refracting  power  of  the  eye  when  in  a 
state  of  complete  repose,  and  when  its  maximum 
amount  of  accommodation  is  in  force,  and  may  be  ex- 
pressed by  the  formula 

a  ■=  p  —  r. 

A  convex  glass  placed  in  front  of  the  eye  produces 
the  same  effect  as  accommodation,  i.e.  it  increases  its 
refraction  and  adapts  the  eye  for  nearer  objects.  We 
can  easily  understand  that  the  lens  which  enables  an 
eye  to  see  at  its  near  point  without  accommodating 
is  a  measure  of  the  amplitude  of  accommodation, 
giving  to  rays  which  come  from  the  near  point  a 
direction  as  if  they  came  from  the  far  point. 

The  amplitude  of  accommodation  is  much  the  same 
in  every  kind  of  refraction.  If  we  wish  to  measure 
it  in  an  emmetrope,  we  have  merely  to  find  the  nearest 
point  at  which  the  patient  can  read  small  print.  A 
lens  whose  focal  distance  corresponds  to  this  is  a 
measure  of  the  amplitude  of  accommodation.     Thus, 


38  THE    EEFEACTION    OF    THE    EYE 

supposing  20  cm.  the  nearest  distance  at  whicli  he 
is  able  to  read  small  prints  we  divide  this  into  100  cm. 
to  find  the  focal  distance  of  the  lens  (-^-^  =  5  D.) ; 
and  in  this  case  a  lens  of  5  D.  is  the  measure  of  the 
amplitude  of  accommodation. 

Or  we  can  measure  it  in  an  inverse  manner  by 
looking  at  a  distant  object  through  a  concave  glass ; 
the  strongest  lens  with  which  we  can  see  this  distant 
object  distinctly  is  the  amplitude  of  accommodation, 
the  concave  lens  giving  to  parallel  rays  coming  from 
the  distant  object  such  an  amount  of  divergence  as 
if  they  came  from  a  point  situated  at  the  principal 
focal  distance  of  this  glass. 

Therefore  the  amplitude  of  accommodation  in 
emmetropia  is  equal  to  the  refraction  when  adapted 
to  its  punctum  proximum,  and  may  be  expressed  by 
the  formula 

a  =  p  —  GO  "^ 
or  a  =  j)  —  0 
or  a  =  p. 

The  Accommodation  of  Hypermetropes. — A  hyper- 
metrope  requires  some  of  his  accommodation  for  dis- 
tant objects ;  we  must  therefore,  in  order  to  find  the 
amplitude  of  accommodation  in  his  case,  add  on  to 
the  lens  whose  focal  length  equals  the  distance  of  the 
near  point,  that  convex  lens  which  enables  him  to  see 
distant  objects  without  his  accommodation,  and  we 
express  it  by  the  formula 

a  =  p  —  (—  r)  =p  +  r. 

Thus^  to   take    an   example,   we   will   assume   the 
*  X  is  the  sign  for  expressing  infinity. 


AMPLITUDE    OF   ACCOMMODATION  39 

patient^s  near  point  to  be  25  cm.  (^^  =  4  J).),  and 
that  lie  has  to  use  4  D.  of  accommodation  for  distant 
objects ;  then  the  amplitude  of  his  accommodation 
would  be  4  D.  +  4  D.  =  8  D. 

a  =  4  D.  -  {-  4^  D.)  =  8  D. 

The  Accommodation  of  Myopes. — In  a  myope  we 
have  to  subtract  the  glass  which  enables  him  to  see 
clearly  distant  objects^  from  that  lens  whose  focal 
length  equals  the  distance  of  the  near  point.  The 
formula  will  then  be 

a  =  p  —  r. 

Thus,  to  find  the  amplitude  of  accommodation  in  a 
myope  of  2  D.,  the  near  point  being  at  10  cm.,  we 
subtract  from  (-y^  =  10)  10  D.  the  amount  of  the 
myopia,  2  D.,  and  the  resulting  8  D.  is  therefore  the 
amplitude  of  accommodation. 

a  =  10  D.  -  2  D.  =  8  D. 
Hence  it  is  obvious  that,  with  the  same  amplitude 
of  accommodation,  the  near  point  is  further  away  in 
hypermetropia  than  in  emmetropia,  and  further  in 
emmetropia  than  in  myopia.  Thus  an  emmetrope, 
with  an  amplitude  of  accommodation  of  5  D.,  would 
have  his  near  point  at  (-^-|-^  =  20)  20  cm. ;  a  hyper- 
metrope  of  2  D.,  whose  amplitude  equalled  5  D.,  would 
require  to  use  2  D.  of  his  accommodation  for  distance, 
leaving  him  3D.,  which  would  bring  his  near  point  to 
(i^  =  33)  33  cm. ;  and  a  myope  of  2  D.,  who  would 
require  a  concave  glass  of  this  strength  to  enable  him 
to  see  at  a  distance,  would  have  a  near  point  of  14  cm. 
(-L^  =14)  with  the  same  amplitude. 


40  THE    REPEACTION    OF    THE    EYE 

Accommodation  is  spoken  of  as  absolute,  hmocular, 
and  relative. 

Absolute  is  the  amount  of  accommodation  wliicli 
one  eye  can  exert  wlien  the  other  is  excluded  from 
vision. 

Binocular,  that  which  the  two  eyes  can  exert  to- 
gether, being  allowed  at  the  same  time  to  converge. 

Relative,  that  which  the  two  eyes  can  exert  together 

for  any  given  convergence  of  the  visual  lines. 

Fig.  31. 
Dioptres. 


Diagram  showing  by  the  number  of  squares  through  which 
the  thick  lines  pass,  the  amplitude  of  accommodation  at 
different  ages  in  emmetropia.  The  figures  above  represent 
the  amount  of  accommodation  ;  those  below,  the  near 
point ;  and  those  on  the  left,  the  age  of  the  individual. 

Fig.  31  diagrammatically  represents  the  amplitude 
of  accommodation  in  ennnetropia. 

As  age  advances  the  elasticity  of  the  lens  dimin- 
ishes, the  accommodation  therefore  becomes  less,  and 
the  near  point  gradually  recedes.  These  changes 
commence  at  a  very  early  age,  long  before  the  indi- 
vidual has  come  to  maturity. 


CONVERGENCE 


41 


The  following  table  gives  the  amplitude  of  accom- 
modation at  different  ages  as  shown  in  Fig.  89, 
p.  188. 

Years.  Amplitude  of  accommodation. 

10 14  D. 

15 12  D. 

20 10  D. 

30 7  D. 

40 4-5  D. 

50 2-5  D. 

60 ID. 

75 0 

Convergence. — This  is  the  remaining  element  of  dis- 
tinct binocular  vision,  and  with  this  function  the 
accommodation  is  very  intimately  linked,  so  that 
usually  for  every  increase  of  the  convergence  a  certain 
increase  in  the  accommodation  takes  place. 

Convergence  is  the  power  of  directing  the  visual 
axes  of  the  two  eyes  to  a  point  nearer  than  infinity, 
and  is  brought  about  by  the  action  of  the  internal 
recti  muscles. 

When  the  eyes  are  completely  at  rest,  the  optic  axes 
are  either  parallel,  or  more  usually  slightly  divergent. 
The  anoxic  thus  formed  between  the  visual  and  the 
optic  axis  is  called  the  angle  a,  and  varies  according 
'to  the  refraction  of  the  eye.  In  emmetropia  the  angle 
is  usually  about  5°;  in  hypermetropia  it  is  greater, 
sometimes  as  much  as  7^  or  8°,  giving  to  the  eyes  an 
appearance  of  divergence ;  in  myopia  the  angle  is 
less,  often  about  2°,  or  the  optic  axis  may,  even  in 
extreme   cases,  fall  on   the  inside  of  the  visual  axis, 


42  THE    REFEACTION    OF    THE    EYE 

when  the  angle  a  is  spoken  of  as  negative  (p.  203)  ; 
so  that  in  myopia  there  is  frequently  an  appearance 
of  convergence^  giving  one  the  idea  of  a  convergent 
squint ;  hence  the  mere  look  of  the  patient^s  eyes  with 
regard  to  their  axes  is  not  always  quite  reliable. 

The  object  of  convergence  is  the  directing  of  the 
yellow  spot  in  each  eye  towards  the  same  point,  so  as 
to  produce  singleness  of  vision ;  diplopia,  or  double 
vision,  at  once  results  when  the  image  of  an  object 
is  formed  on  parts  of  the  retina  which  do  not  exactly 
correspond  in  the  two  eyes. 

To  test  the  power  of  convergence  prisms  are  held 
with  their  bases  outwards.  The  strongest  prism  which 
it  is  possible  to  overcome,  that  is  the  prism  which 
does  not  produce  diplopia  on  looking  through  it 
at  a  distant  object,  is  the  measure  of  the  converg- 
ence, and  varies  in  different  persons,  usually  between 
prisms  of  20°  and  30°,  divided  between  the  two  eyes. 
This  is  the  relative  convergence  for  infinity. 

In  considering  convergence  we  have  not  only  to 
bear  in  mind  the  condition  of  the  internal  recti 
muscles,  but  also  the  state  of  equilibrium  produced 
by  them  and  the  action  of  their  antagonists — the 
external  recti. 

The  nearer  the  object  looked  at,  the  more  we  have 
to  converge,  and  the.  greater  the  amount  of  accom- 
modation brought  into  play.  Hence,  on  converging 
to  any  particular  point,  we  usually  also  involuntarily 
accommodate  for  that  point,  the  internal  recti  and 
ciliary  muscles  acting  in  unison. 

Nagel  has  proposed  a  very  simple  and  convenient 


CONVEEGENCE 
Fig.  32. 


43 


44  THE    REFRACT] ON    OF    THE    EYE 

plan,  by  wliicli  we  may  express  tlie  convergence  in 
metres,  calling  the  angle  formed  by  the  visual  and 
median  lines,  as  at  m',  the  metrical  angle.  In  Fig.  32 
E,  e'  represent  the  centres  of  rotation  for  the  two  eyes ; 
E  H  e'  is  the  base  line  between  the  centres.  When 
the  eyes  are  fixed  on  some  distant  object,  the  visual 
lines  are  parallel  or  almost  so,  as  e  a,  e'  a'  ;  the 
angle  of  convergence  is  then  at  its  minimum,  and 
the  convergence  is  said  to  be  adapted  for  its 
pionctum  remotum ;  this  then,  being  at  infinity,  is 
expressed  C  ^  =  oo  . 

If  the  eyes  be  directed  to  an  object  one  metre 
away,  the  metrical  angle  E  m'  h  equals  one,  i.  e.C  =  J . 
If  the  object  is  50  cm.  olf,  then  0  =  2;  if  10  cm., 
then  (Y/  =  10;  C  =  10.  If  the  object  had  been  be- 
yond 1  metre  (our  unit),  but  not  at  infinity,  say  4 
metres,  then  C  =  ^. 

When  the  visual  lines,  instead  of  being  parallel, 
diverge,  then  the  punctum  remotum  is  found  by  con- 
tinuing these  lines  backwards  till  they  meet  at  c, 
behind  the  eye  ;  the  convergence  is  then  spoken  of 
as  negative. 

When  the  eyes  are  directed  to  the  nearest  point  at 
which  they  can  see  distinctly,  say  at  m'",  the  angle  of 
convergence  is  at  its  maximum,  and  it  is  said  to  be 
adapted  for  its  ininctum  lyroAmiim. 

The  distance  between  the  punctum  proximum  and 
the  punctum  remotum  is  the  range  of  convergence. 

The  amplitude  of  convergence  is  the  whole  converg- 

*  C  is  the  sign  for  convergence. 


CONVEEGENCE  45 

ence  that  can  be  put  in  force,  and  we  express  it  by 
the  formula. 

c  =  p  —  r. 

The  functum  remotum  of  convergence  is  seldom 
situated  at  a  finite  distance  :  sometimes  it  is  exactly 
at  infinity,  but  in  the  majority  of  cases  it  is  situated 
beyond  infinity,  i.  e.  the  visual  lines  diverge  slightly. 
In  order  to  measure  this  divergence,  and  so  obtain 
the  punctum  remotum  of  convergence,  we  place  before 
the  eyes  prisms  with  their  bases  inwards  (abducting 
prisms),  and  the  strongest  prism  through  which  the 
person  is  still  able  to  see  singly  is  the  measure  of  the 
negative  convergence. 

Prisms  are  numbered  in  degrees  according  to  the 
angle  of  the  prism.  The  deviation  produced  by  a 
prism  is  equal  to  half  its  angle ;  thus  prism  8  will 
produce  a  deviation  of  the  eye  of  4°,  and  prism  20  a 
deviation  of  10°. 

When  a  prism  is  placed  before  one  eye,  its  action 
is  equally  divided  between  the  two  eyes. 

To  take  an  example  :  if  an  abducting  prism  of  8° 
placed  before  one  eye  (or  what  is  the  same  thing,  4° 
before  each  eye)  be  found  to  be  the  strongest  through 
which  a  distant  object  can  be  seen  singly,  then  each 
eye  in  our  example  has  made  a  movement  of  diverg- 
ence equal  to  2°,  and  the  punctum  remotum  of  con- 
vergence in  this  case  is  therefore  negative,  and  is  ex- 
pressed —  2°.  By  referring  to  the  table  on  page  49 
it  will  be  seen  that  when  the  centres  of  rotation  of 
the    eyes    are   6*4  cm.   apart,  then  the  metre    angle 


46  THE    REFRACTION    OP    THE    EYE 

equals  1°  50',  so  we  reduce  the   2°  to  metre  angles, 

thus  : 

2^       120      ^ 
p^,=: j-^^=  1-09  m  a; 

or  it  is  sufficient  to  remember  to  divide  the  prism 
placed  before  one  eye  by  seven ;  thus  in  our  example 
we  should  divide  prism  8°  by  seven,  and  this  would 
give  us  approximately  the  same  result. 

Another  excellent  plan  for  finding  the  punctum 
remotum  of  convergence  is  by  Maddox^s  test,  which 
consists  of  a  small  glass  rod  placed  behind  a  stenopaic 
slit ;  when  this  is  held  horizontally  before  the  right 
eye,  and  the  flame  of  a  candle  viewed  from  a  distance 
of  6  metres  with  both  eyes  open,  the  left  eye  receives 
the  image  of  the  flame,  while  the  right  receives  the 
image  which  is  drawn  out  by  the  rod  into  a  long 
vertical  strip  of  light ;  and  since  the  image  received 
by  the  two  eyes  is  very  different,  there  is  no  tendency 
to  fusion,  and  the  eyes  take  up  their  position  of  rest. 
A  suitable  scale  placed  behind  the  candle  will  give  us 
the  amount  of  convergence  or  divergence  in  metre 
angles,  according  to  the  position  occupied  by  the 
streak  of  light  on  the  scale.  Should  the  patient  be  a 
myope  or  a  hypermetrope  he  should  wear  his  correc- 
tion when  this  test  is  applied. 

To  find  the  punctum  proximum  of  convergence,  hold 
a  prism,  base  outwards  (adducting  prism),  before  one 
eye,  and  the  strongest  prism  which  can  be  so  employed 
without  producing  diplopia,  divided  between  the  two 
eyes,  gives  the  punctum  proximum  of  convergence  in 
degrees.     But  the  accommodation  must  be  stimulated 


CONVERGENCE 


47 


at  the  same  time  by  means  of  a  concave  glass,  other- 
wise we  obtain  only  the  relative  punctum  proximum. 
This  can  be  reduced  to  metre  angles  as  before. 

Or  a  simpler  plan  is  to  measure  it  with  Landolt's 
ophthalmo-dynamometer,  which  is  a  small  instrument 
consisting  of  a  black  metallic  cylinder,  a,  made  so  as 
to  fit  upon  a  candle,  b.  The  cylinder  has  a  vertical 
slit  '3  mm.  in  breadth,  covered  by  ground  glass :  the 
candle  being  lighted,  this  slit  forms  a  luminous  line, 

Fig.  33. 


and  will  serve  as  a  fixation  object.  A  tape  measure 
is  conveniently  attached,  being  graduated  in  centi- 
metres on  one  side,  and  on  the  other  in  the  corre- 
sponding numbers  of  metre  angles. 

To  find  the  punctum  proximum  of  convergence,  the 
measure  is  drawn  out  to  about  70  cm.,  its  case  being 
held  beside  one  of  the  eyes  of  the  patient,  while  the 
object  of  fixation  is  placed  in  the  median  line.  If  the 
illuminated  line  is  seen  singly,  by  pressing  the  knob 


48  THE    REFRACTION    OF    THE    EYE 

of  the  case  the  spring  rolls  up  the  tape^  and  the  fixa- 
tion object  is  brought  nearer  the  eye.  So  soon  as  the 
patient  commences  to  see  double,  the  nearest  point  of 
convergence  is  obtained,  and  the  maximum  of  con- 
vergence is  read  off  the  tape  in  metre  angles.  This 
experiment  should  be  repeated  several  times. 

In  a  normal  case  the  minimum  of  convergence  is 
usually  about  —  1  7n  a,  the  maximum  9"5  m  a;  so  that 
the  amplitude  of  convergence  equals  10*5  m  a. 

We  know  that  the  accommodation  increases  the 
nearer  the  object  approaches,  hence  we  see  that  both 
the  convergence  and  accommodation  increase  and 
decrease  together ;  and  in  recording  the  convergence 
in  the  manner  proposed  by  Nagel,  it  will  be  seen  that 
in  the  emmetropic  eye  the  number  which  expresses 
the  metrical  angle  of  convergence  expresses  also  the 
state  of  refraction  for  the  same  point — this  is  a  great 
advantage.  Thus,  when  looking  at  a  distant  object, 
the  angle  of  convergence  is  at  infinity,  C.  =  oo  ;  and 
the  refraction  is  also  at  infinity,  A  =  go  .  When  the 
object  is  at  1  metre,  the  angle  of  convergence  =  1, 
and  the  amount  of  accommodation  put  into  play 
=  1  D.  When  the  object  is  at  25  cm.,  then  the  angle 
of  convergence  =  4,  and  the  amount  of  accommoda- 
tion =  4  D. 

The  amplitude  of  convergence  is  somewhat  greater 
than  the  amplitude  of  accommodation,  passing  it  both 
at  its  punctum  remotum  and  its  punctum  proximum. 

The  following  table  shows  the  angle  of  convergence 
in  degrees,  for  different  distances  of  the  object,  when 
the  eyes  are  6*4  cm.  apart: 


CONVERGENCE 

•istance  of  the  object 
from  the  eyes. 

The  metrical 
angle. 

1  metre 

1 

50  cm. 

2 

33    „ 

3 

25    „ 

4 

20    „ 

5 

16    „ 

6 

14    „ 

7 

12    „ 

8 

11    „ 

9 

10    „ 

10 

9    „ 

11 

8    „ 

12 

7-5„ 

13 

7   „ 

14 

6-5„ 

15 

6    „ 

16 

5-5„ 

18 

5    „ 

20 

49 


Value  expressed 
in  degrees. 

1=50' 

3°  40' 

5°  30' 

7°  20' 

9°  10' 
11° 

12"  50' 
14°  40' 
16°  30' 
18°  20' 
20=  10' 
22° 

23°  50' 
25°  40' 
27°  30' 
29°  20' 
33° 
36°  40' 


Althougli  accommodation  and  convergence  are  thus 
intimately  linked  together,  it  can  very  easily  be 
proved  that  they  may  have  a  separate  and  indepen- 
dent action.  If  we  suspend  the  accommodation  with 
atropine,  the  convergence  is  not  interfered  with ;  or 
an  object  at  a  certain  distance  being  seen  clearly 
without  a  glass,  it  can  still  be  seen  distinctly  with 
weak  concave  and  convex  glasses,  without  any  altera- 
tion of  the  convergence. 

It  may,  therefore,  be  stated  that  although  the 
accommodation  and  convergence  are  intimately  asso- 
ciated, they  may  be  independent  of  each  other  to  a 
certain  degree,  so  as  to  meet  ordinary  requirements  ; 
thus  for  instance,  as  age  advances  changes  take  place 

4 


50 


THE    KEFRACTION    OF    THE    EYE 


in  the  lens  wliicli  necessitate  a  stronger  contraction  of 
the  ciliary  muscle  to  produce  the  requisite  change  in 
the  accommodation^  while  the  convergence  remains 
the  same. 

It  is  obvious  also  that  the  relations  between  accom- 
modation and  convergence  must  necessarily  be  very 
different  in  ametropia^  and  this  relation  will  be 
agrain  referred  to  when  treatino-  the  various  errors 
of  refraction  in  detail. 

The  following  diagram  (Fig.  34)  shows  the  relative 
amount  of  accommodation  for  different  points  of  con- 

FiG.  34. 


vergence  in  an  ennnetrope  aged  fifteen.     The  amount 
of  accommodation  in  excess  of  the  metrical  angle  of 


RELATIVE    ACCOMMODATION  51 

convergence  is  called  iiositire,  and  tlie  amount  below 
negative. 

The  diagonal  d  d  represents  the  convergence  from 
infinity  to  5  cm. ;  it  is  also  a  record  of  the  accommo- 
dation. The  line  p  p'  p"  indicates  the  maximum  ac- 
commodation for  each  point  of  convergence^  and  the 
line  r  v  the  minimum.  The  numbers  on  the  left 
and  below  the  diagram  are  dioptres  and  metrical 
angles  of  convergence ;  thus,  when  the  visual  lines 
are  parallel,  it  will  be  seen  that  3'5  D.  of  positive 
accommodation  can  be  put  into  play,  i.  e.  the  object 
can  still  be  seen  distinctly  with  a  concave  glass  of  that 
strength ;  3'5  D.  is  therefore  the  relative  amplitude  of 
accommodation  for  convergence  adapted  to  infinity ; 
or  the  metrical  angle  C  being  5,  which  is  a  distance 
of  20  cm.  away,  the  accommodation  for  that  point 
would  equal  5  D. ;  the  positive  amount  that  can  be 
put  in  force  while  the  angle  of  C  remains  the  same 
would  be  3  D.,  and  the  negative  also  3  D.,  the  object 
being  seen  clearly  with  a  concave  or  convex  glass  of 
3  D.,  therefore  the  relative  amplitude  of  accommoda- 
tion for  C  5  is  6  D.  When  the  angle  C  =  10  or  any 
larger  angle,  the  accommodation  that  can  be  put  in 
force  will  be  seen  to  be  entirely  on  the  negative  side. 

Thus,  the  convergence  being  fixed,  the  amount  of 
accommodation  which  can  be  brought  into  play  for 
that  particular  point  is  the  sum  of  the  diiference 
between  the  strongest  concave  and  convex  glass 
employed. 

The  eye  being  accommodated  for  an  object  at  a 
certain  distance,  the  amount  of  convergence  for  that 


52  THE    REFRACTION    OP    THE    EYE 

particular  point  may  be  measured  by  placing  in  front 
of  the  eyes  prisms,  bases  outwards;  the  strongest 
prism  through  which  the  object  is  still  seen  singly 
is  the  measure  of  the  positive  part  of  the  amplitude 
of  convergence.  Prisms,  bases  inwards,  give  us  the 
negative  part — the  sum  of  these  is  the  amplitude  of 
relative  convergence. 


METHODS    OF    DETERMINING    THE    EEFRACTION  53 


CHAPTER  III 

METHODS    OF    DETERMINING   THE    REFRACTION 

In  entering  upon  the  practical  part  of  the  subject 
the  following  subjective  and  objective  methods  present 
themselves  for  consideration. 

1.  The  acuteness  of  vision. 

2.  Scheiner's  method. 

3.  The  ophthalmoscope. 

{a)   The  indirect  method. 

(b)  The  direct  method. 

(c)  Retinoscopy. 

In  every  case  we  must  proceed  in  a  systematic 
manner^  and  before  commencing  to  take  the  patient^s 
visual  acuteness^  something  may  be  gained  by  noticing 
the  general  appearance  of  the  patient,  the  form  of  the 
face,  head,  etc. ;  thus  a  flat-looking  face  is  sometimes 
an  indication  of  hypermetropia ;  a  head  elongated  in 
its  antero-posterior  diameter,  with  a  long  face  and 
prominent  nose,  may  indicate  myopia.  If  the  two 
sides  of  the  face  are  not  symmetrical,  or  if  there  be 
some  lateral  displacement  of  the  nose  from  the  median 
line,  astigmatism  may  be  suspected.  We  should  also 
notice  the  shape  of  the  eyes  themselves,  if  large  and 
prominent,  or  small;  in  the  former  case  we  may  sus- 
pect myopia,  in  the  latter  hypermetropia.  Large 
pupils  are  suggestive  of  myopia,  and  small  pupils  of 


54  THE    REFRACTION    OF    THE    EYE 

liypermetropia.  In  liigli  degrees  of  astigmatism  it  can 
sometimes  be  seen  that  tlie  curvature  of  tlie  cornea  in 
one  meridian  exceeds  that  of  the  other.  The  distance 
between  the  eyes  should  also  be  noted^  as  well  as  the 
direction  of  their  visual  axes. 

We  next  listen  to  the  patient's  own  statement  of  the 
troubles  from  which  he  suffers ;  he  may  say  that  he 
sees  distant  objects  well  but  has  difficulty  in  reading, 
especially  in  the  evenings,  or  that  after  reading  for 
some  time  the  type  becomes  indistinct,  so  that  he  must 
rest  awhile, — here  we  suspect  hypernietropia ;  or  he 
may  be  able  to  read  and  do  near  work,  but  sees  badly 
at  a  distance, — then  we  suspect  myopia;  or  both  near 
and  distant  vision  may  be  defective, — in  this  case  our 
first  object  must  be  to  decide  whether  the  imperfect 
vision  is  due  to  some  error  of  refraction  or  to  some 
structural  change  in  the  eyes  themselves ;  and  we 
possess  an  extremely  simple  method  by  which  to 
differentiate  between  them,  and  this  method  is  called 
the  Pin-hole  test. 

Pin-hole  Test. — A  black  diaphragm  having  a  small 
perforation  in  its  centre  (the  box  of  trial  glasses 
usually  .contains  such  a  diaphragm)  is  placed  quite 
close  to  the  eye  under  examination.  This  perforation 
gives  passage  to  a  small  pencil  of  rays  which  passes 
through  the  axis  of  the  refracting  system  of  the  eye, 
so  that  the  image  formed  is  clearly  defined  for  all 
distances  :  if  then  the  pin-hole  improve  vision,  the 
refractive  system  is  at  fault;  but  if,  on  the  contrary, 
vision  is  not  improved,  then  we  suspect  that  the 
transparency  of  the  media  or  that  the  retinal  sensi- 


ACUTENESS    OF    VISION  55 

bility  is  defective ;  thus  we  possess  a  very  simple  and 
reliable  plan,  which  if  used  systematically,  may  save 
much  loss  of  time.  The  points  to  notice  when  apply- 
ing this  test  are,  that  the  illumination  is  good,  and 
that  the  pin-hole  is  immediately  in  front  of  the  centre 
of  the  pupil. 

Having  then  found  out  that  the  patient's  refraction 
is  defective,  we  proceed  to  the  first  method,  the 
acuteness  of  vision. 

The  Acuteness  of  Vision. — This  must  not  be  confused 
with  the  refraction,  and  it  is  necessary  clearly  to 
understand  the  difference  between  these  two  terms. 
The  acuteness  of  vision  is  the  function  of  the  nervous 
apparatus  of  the  eye,  while  the  refraction  is  the  func- 
tion of  the  dioptric  system;  so  that  the  acuteness  of 
vision  may  be  normal,  even  if  the  refraction  be  very 
defective,  provided  it  has  been  corrected  by  glasses. 
The  refraction,  on  the  other  hand,  may  be  normal, 
even  though  the  eye  is  unable  to  see,  as  in  cases  of 
optic  atrophy,  etc. 

We  may  define  the  acuteness  of  vision  as  that  degree 
of  sight  which  an  eye  possesses  after  any  error  of  its 
refraction  has  been  corrected,  and  the  glasses  neces- 
sary for  this  correction  are  therefore  a  measure  of  the 
error  of  refraction. 

In  order  to  find  out  the  acuteness  of  vision,  it  is 
necessary  to  determine  the  smallest  retinal  image  the 
form  of  which  can  be  distinguished  in  the  normal  eye; 
it  has  been  discovered  by  experiments  that  the  smallest 
distance  between  two  points  on  the  retina  which  can 
be  separately  perceived  is  0"00436  mm.,  about  twice 


56  THE    REFRACTION    OF    THE    EYE 

the  diameter  of  a  single  cone;  but  it  is  only  at  the 
macula  and  the  part  immediately  around  it^  Avhich  is 
the  most  sensitive  part  of  the  retina_,  that  the  cones 
are  so  close  together  as  "002  mm. ;  in  the  periphery 
of  the  field  of  vision  the  two  points  must  be  further 
apart  to  appear  distinct. 

It  is  probable  that  rays  from  two  points  must  fall 
upon  two  diiferent  cones  in  order  to  be  visible  as  two 
distinct  objects. 

The  same  thing  may  be  expressed  in  another  way ; 
the  smallest  retinal  image  which  can  be  perceived  at 
the  macula  corresponds  to  a  visual  angle  of  V,  so  that 
two  stars  separated  by  an  angular  interval  of  less 
than  r  would  produce  upon  the  eye  the  eifect  of  one 
star  only. 

The  visual  angle  has  been  shown  to  be  an  angle 
included  between  two  lines  drawn  from  the  two  oppo- 
site edges  of  the  object  through  the  nodal  point  (Figs. 
28  and  35). 

Fig.  35. 


Test-types  have  been  constructed  upon  these  prin- 
ciples for  determining  the  acuteness  of  vision,  Snel- 
ling's  being  those  ordinarily  used.  Every  letter  is  so 
made  that  when  at  its  proper  distance,  each  part  of  it 


ACUTENESS    OF    VISION  57 

is  separated  from  the  other  parts  by  an  interval  equal 
to  not  less  than  the  arc  subtending  an  angle  of  1'  at 
the  nodal  pointy  while  the  whole  letter  subtends  an 
angle  of  5'. 

In  order  to  estimate  the  refraction  by  the  acuteness 
of  vision,  the  test  object  must  be  placed  in  a  good 
light,  and  so  far  away  as  to  exclude  as  much  as  pos- 
sible the  accommodation, — 6  metres  has  been  found  to 
be  a  sufficient  distance ;  rays  coming  from  an  object 
so  far  off  may  be  assumed  to  be  parallel,  and  falling 
on  an  emmetropic  eye  at  rest,  come  to  a  focus  on  the 
retina.  The  smallest  letter  which  can  be  seen  distinctly 
at  this  distance  will  represent  the  patient^s  vision. 

Snellen^s  type  consists  of  rows  of  letters,  each  row 
being  marked  above  with  the  distance  in  metres  at 
which  it  should  be  read.  The  top  letter  should  be 
distinct  at  60  metres,  the  next  at  36,  and  each  succeed- 
ing row  at  24,  18,  12,  9,  and  6  metres  respectively."^ 
The  patient  placed  at  six  metres  should,  without  any 
accommodation,  be  able  to  read  the  bottom  line  with 
either  eye.  This  is  expressed  in  the  form  of  a  fraction, 
in  which  the  numerator  indicates  the  distance  at  which 
it  is  read,  and  the  denominator  the  number  of  the  line. 
We  note  down  the  result  found  for  each  eye  sepa- 
rately :  if  the  bottom  line  is  read,  |-  expresses  it ;  if 
the  next,  |-;  the  top,  -^,  etc. 

If  our  patient  is  not  able  even  to  see  the  large  letter 
at  the  top,  we  allow  him  to  approach  the  board,  telling 

*  The  set  of  test-types  at  the  end  of  the  book  has  two  additional 
lines,  marked  5  and  4,  so  that  a  greater  amount  of  visual  acute- 
ness than  f  can  be  estimated,  and  is,  of  course,  recorded  |  and  f . 


58  THE    REFRACTION    OF    THE    EYE 

him  to  stop  when  he  recognises  the  letter.  Supposing 
he  stop  at  two  metres  from  the  board,  we  express  that 
as  -^^^ ;  if  he  is  not  able  to  read  it  when  quite  close  to 
the  eye,  we  see  how  far  off  he  can  count  fingers.  If 
unable  to  do  this,  the  hand  may  be  passed  quickly  in 
front  of  the  eye,  and  if  these  movements  are  seen 
the  vision  is  expressed  "hand  movements^'  (H.  M.). 
Should  these  movements  not  be  seen  we  throw  light 
into  the  eye  with  a  convex  lens ;  if  the  light  is  per- 
ceived, it  is  called  perception  of  light  (P.  L.).  When 
the  patient  fails  to  distinguish  the  difference  between 
light  and  darkness  the  eye  is  quite  blind,  there  is  no 
perception  of  light  (no  P.  L.). 

Although  the  capability  of  reading  the  bottom 
line  at  6  metres  is  the  average  acuteness  of  vision, 
yet  it  is  not  the  maximum,  since  many  young  people 
will  be  found  who  are  able  to  read  line  six  at  7 
metres,  or  even  further,  in  which  case  their  acute- 
ness is  I". 

Savages  often  have  an  acuteness  of  vision  much  in 
excess  of  the  normal. 

Thus  we  have  a  standard  of  normal  vision,  and  a  con- 
venient method  of  expressing  it  in  a  numerical  manner. 

We  put  our  patient  then,  with  his  back  to  the  light, 
in  front  of  the  test-types,  which  must  hang  well  illu- 
mined at  6  metres  distance,  and  having  armed  him 
with  a  pair  of  trial  frames,  we  exclude  the  left  eye 
from  vision  by  placing  in  front  of  it  an  opaque  disc, 
and  proceed  to  test  the  right  eye  by  asking  him 
how  much  of  the  type  he  is  able  to  read ;  if  he  read 
the  line  marked  6,  then  his  vision  is  ^  or  1,  that  is  to 


ACUTENESS    OF    VISION  59 

say,  his  distant  vision  is  normal ;  we  may,  therefore, 
assume  the  absence  of  myopia  or  astigmatism;  but 
he  may  have  hypermetropia,  and  only  be  able  to  read  -| 
by  using  his  accommodation  ;  this  we  decide  by  hold- 
ing a  weak  convex  glass  (+  '5  D.)  in  front  of  the  eye, 
when  if  he  still  be  able  to  read  the  same  line  f,  he 
has  hypermetropia,  and  the  strongest  convex  glass 
with  which  #  can  be  read  is  the  measure  of  the  mani- 
fest  hypermetropia ;  supposing  +  1  D.  the  strongest 
glass  with  which  -J  can  be  read,  then  we  record  it 
thus :  R.  V.  f  Hm.   1  D.  =  f . 

I  say  manifest  hypermetropia,  because  in  all  cases 
occurring  in  young  people  this  is  not  the  total  hyper- 
metropia; for  in  these  a  great  part  of  the  error  is  latent, 
and  can  only  be  discovered  by  using  atropine,  or  by 
estimating  the  refraction  by  the  direct  opthalmo- 
scopic  method.  Many  cases  will  come  before  us 
having  two  or  three  dioptres  of  hypermetropia,  who 
complain  that  the  weakest  convex  glass  impairs 
distant  vision ;  in  these  cases  the  hypermetropia  is 
wholly  latent. 

We  may  say,  therefore,  that  a  patient  w^ho  is  able 
to  read  |-  with  one  eye,  must  be — 
Emmetropic 
or 
Hypermetropic  in  that  eye. 

If  hypermetropic,  a  part  of  the  defect  is  usually 
manifest ;  the  strongest  convex  glass  which  does  not 
impair  distant  vision  gives  us  the  amount.  If  the 
hypermetropia  is  wholly  latent,  then  it  is  necessary 
to  atropise  the  patient  before  it  can  be  demonstrated. 


60  THE    REFRACTION    OF    THE    EYE 

Supposing^  however,  our  patient^s  vision  is  below 
the  normal,  and,  instead  of  reading  |-,  he  is  only  able 
to  read,  say  the  third  line  (2^^),  and  that  this  is 
blurred  with  a  weak  convex  glass,  he  may  have — 

Myopia, 

Astigmatism^ 
or 

Spasm  of  the  accommodation   (see  p.   197). 
We  try  if  a  weak  concave  glass  helps  him ;  if  it  does 
so^  the  case  is  one  of  myopia  ;  and  we  find  the  iveakest 
concave  glass  with  which  he  sees  best ;  thus  to  take 
an  example  in  which  the  patient  is  a  myope  and  sees 
only  YT^  but  with  —  2D.  reads  f ;  we  repeat  the  exami- 
nation with  the  second  eye,  and  record  it — 
R.V.J__2D.  =f. 
L.V.^\-2D.  =|. 
If  the  patient  is  not  improved  with  concave  glasses, 
then   we    assume    that    some  astigmatism  is  present, 
presupposing  of  course  that  there  is  no  other  cause 
for  bad  vision. 

To  estimate  this  astigmatism  we  must  call  to  our 
aid  some  of  the  methods  described  in  the  chapter  on 
astigmatism,  p.  156,  or  first  find  out  the  spherical 
glass  with  which  he  is  able  to  see  best,  then  rotate  in 
front  of  it  a  weak  convex  cylindrical  glass,  starting 
with  its  axis  vertical ;  no  improvement  occurring,  do 
the  same  with  a  weak  concave  cylinder,  starting  with 
its  axis  horizontal ;  finding  by  this  plan  the  glass  and 
its  particular  axis  which  gives  the  best  result.  It 
is  necessary  that  the  eye  be  thoroughly  under  the 
influence  of  atropine,  in  order  to  enable  us  to  arrive 


ACUTENESS    OE    VISION  61 

at  definite  and  reliable  results  by  this  method.  With 
practice,  one  is  able  in  this  way  to  work  out  simple 
cases  of  astigmatism  accurately  and  quickly. 

The  object  in  view  is  always  to  bring  up  the  vision 
of  each  eye  as  nearly  to  the  normal  standard  (-f)  as 
possible.  Frequently,  however,  we  have  to  be  satisfied 
with  |-  or  -j\. 

But  should  the  case  appear  to  be  a  difficult  one,  it 
is  better  perhaps  for  the  student  not  to  waste  time, 
but  proceed  at  once  to  retinoscopy. 

When  trying  the  patient  at  the  distant  type  it  is 
convenient  to  have  two  or  more  sets  of  letters,  so  that 
the  type  may  be  changed  when  the  patient  gets  accus- 
tomed to  one  set. 

The  near  type  is  chiefly  used  to  estimate  the 
accommodation,  by  finding  out  the  far  and  near  point 
at  which  any  particular  line  is  read.  Snellen^s  and 
Jaeger^s  are  the  types  most  commonly  in  use,  many 
preferring  Jaeger's,  owing  to  the  letters  being  of  the 
ordinary  shapes;  but  they  have  the  disadvantage  that 
they  are  not  arranged  on  any  scientific  plan,  but  are 
simply  printer's  types  of  various  sizes :  the  set  of 
reading  type  at  the  end  of  the  book  is  so  arranged 
that  when  held  at  the  distance  for  which  it  is  marked, 
each  letter  subtends  an  angle  of  5'  at  the  nodal 
point. 

It  must,  however,  be  remembered  that  sentences  are 
an  inferior  test  to  letters,  many  people  recognising 
the  words  by  their  general  appearance,  whereas  they 
may  be  unable  to  see  distinctly  every  letter  of  which 
each  word  is  composed. 


62  THE    REFEACTION    OF    THE    EYE 

Having  tested  our  patient's  vision  at  the  distant 
type  and  recorded  the  result,  we  place  in  his  hand  the 
reading  type,  and  note  the  smallest  print  he  is  able  to 
read  and  the  distance  at  which  he  reads  it ;  first  w4th 
each  eye  separately,  then  with  the  two  together. 

In  cases  of  myopia  we  may  thus  get  a  valuable 
hint  as  to  the  amount  of  the  defect :  take  for  an  ex- 
ample a  case  in  which  the  patient  can  read  -J^-^  with 
the  right  eye;  we  give  him  the  near  type,  and  if 
he  can  read  the  smallest  only  by  holding  it  at  a 
nearer  point  than  the  distance  for  which  it  is  marked, 
note  the  greatest  distance  at  which  he  is  able  to 
read  it;  if  the  type  marked  for  1  metre  cannot  be  read 
further  off  than  25  cm.,  our  patient  has  then  most 
likely  myopia  of  4  D.,  because  25  cm.  is  probably  his 
far  point.  In  this  case  a  glass  —  4  D.  will  give  to 
rays  coming  from  a  distant  point  the  same  amount  of 
divergence  as  if  they  came  from  25  cm.  (-y^  =  4). 

We  try  the  patient  at  the  distant  type  with  —  4 
D.;  if  he  now  read  -|  the  myopia  is  confirmed,  and  the 
weakest  glass  with  which  he  reads  it  is  the  measure 
of  his  myopia. 

If  the  patient  read  -J,  but  be  unable  to  read  the  near 
type  except  it  be  held  at  a  further  distance  than  that 
for  which  it  is  marked,  the  case  is  one  of  paralysis  of 
the  accommodation,  or  presbyopia;  and  as  the  latter 
only  commences  in  emmetropia  about  the  age  of  forty- 
five,  it  will  be  clear  according  to  the  age  of  the  patient 
to  which  division  the  case  belongs. 

As  objects  seen  through  a  convex  lens  appear  en- 
larged,   and    through    a  concave  lens  diminished,  it 


ACUTENESS    OF    VISION  63 

follows  thatj  when  placed  before  the  eye,  they  will 
produce  the  same  effect.  Now  the  hypermetropic  eye 
sees  objects  smaller  and  the  myopic  eye  larger  than 
the  emmetrope,  and  if  glasses  which  are  to  correct 
the  ametropia  be  placed  in  the  anterior  focal  plane, 
i.  e.  about  13' 7  mm.  in  front  of  the  cornea,  the  retinal 
image  of  the  ametrope  will  be  of  the  same  size  as 
that  of  the  emmetrope. 

Before  leaving  this  subject  of  the  acuteness  of 
vision  the  following  directions  may  be  given : 

1st.  Thd  test-type  must  be  in  a  good  light;  the 
advantage  of  artificial  illumination  is  that  it  is  uni- 
form. 

2nd.  Commence  with  the  right  eye,  or  that  Avhich 
has  the  best  vision,  covering  up  the  other  with  an 
opaque  disc  placed  in  a  spectacle  frame ;  do  not  be 
contented  to  allow  the  patient  to  close  one  eye,  as  he 
may  not  do  so  completely,  or  he  will  probably  uncon- 
sciously slightly  diminish  the  palpebral  aperture  of 
the  eye  under  examination,  whereby  the  circles  of 
diffusion  may  be  somewhat  diminished  and  so  give 
misleading  results.  Neither  should  he  close  the  eye 
with  his  hand,  he  may  look  between  the  fingers,  or 
exercise  some  pressure,  however  slight,  on  the  eye- 
ball, which  may  interfere  temporarily  with  the  func- 
tion of  the  retina  and  so  cause  delay. 

3rd.  Having  noticed  what   each  eye  sees  without 

glasses,  always  begin  the  examination  with  convex  ones, 

so  as  to  avoid  calling  the  accommodation  into  action. 

4th.     Having  recorded  the  result  found  for  each 

eye  separately,  try  the  two  together,  the  binocular 


64  THE    REFEACTION    OF    THE    EYE 

visual  acuteness  being  usually  slightly  greater  than 
that  for  one  eye. 

5th.  Test  the  patient  with  the  reading  type,  noting 
the  nearest  and  farthest  point  at  which  the  smallest 
type  can  be  read. 

Schemer's  Method. — Although  this  plan  for  detecting 
ametropia  is  now  but  little  used,  it  is  necessary  the 
student  should  understand  the  principles  upon  which 
it  is  based.  A  diaphragm  having  two  small  perfora- 
tions is  placed  in  front  of  the  eye  we  wish  to  examine ; 
the  perforations  must  be  so  near  together  that  rays 
passing  through  them  will  enter  the  pupil  (Fig.  86). 
The  patient  is  directed  to  look  at  a  small  flame  6 
metres  off;  rays  emanate  from  this  flame  in  all  direc- 
tions, some  fall  on  the  diaphragm,  the  greater  number 
are  thus  cut  off,  but  a  few  rays  pass  through  the  two 
openings,  and  if  the  eye  be  adapted  for  the  flame, 
^.  e.  if  it  is  emmetropic,  these  two  sets  of  rays  will 

Fig.  36.* 


meet  exactly  on  the  retina,  forming  there  one  image 
of  the  flame   (b.  Fig.  36)  ;    if,  however,  the   eye  be 

*  In  the  above  diagram,  p  is  represented  as  a  near  object  -with 
rays  diverging  from  it ;  it  should  bo  a  distant  object  with  parallel 
rays. 


65 

hypermetropic  (with  suspended  accommodation),  then 
the  two  sets  of  rays  will  reach  the  retina  before  meeting, 
each  set  forming  an  image  of  the  flame  (a_,  Fig.  36). 
The  greater  the  hypermetropia  the  further  apart  will 
the  images  be  formed ;  these  are  projected  outwards 
as  crossed  images,  and  the  patient  has  therefore 
crossed  diplopia.  That  convex  glass  (from  our  trial 
box)  which,  held  behind  the  diaphragm,  causes  the 
flame  to  be  seen  singly,  is  a  measure  of  the  hyperme- 
tropia. If  the  eye  be  myopic,  then  the  two  sets  of 
rays  will  have  crossed  and  are  diverging  when  they 
reach  the  retina,  where  two  images  of  the  flame  are 
therefore  formed  (c.  Fig.  36).  These  images  are  crossed 
again  as  they  are  projected  outwards,  and  having 
twice  crossed,  homonymous  images  are  the  result. 
To  find  the  amount  of  myopia,  we  have  only  to  find 
the  concave  glass  which,  placed  behind  the  diaphragm, 
brings  the  two  images  into  one. 

To  enable  us  to  tell  if  the  images  are  crossed  or 
homonymous,  it  is  usual  to  have  in  front  of  one  of  the 
perforations  a  piece  of  coloured  glass.  We  will  sup- 
pose the  diaphragm  held  so  that  the  two  openings 
are  horizontal,  that  to  the  patient^s  right  having  in 
front  of  it  a  piece  of  red  glass  :  if  only  one  flame  is 
seen  the  case  is  one  of  emmetropia ;  if  two  images  of 
it  appear,  one  white,  the  other  red,  with  the  red  to 
the  left  of  the  other,  the  images  are  crossed,  and  the 
case  is  one  of  hypermetropia.  If  the  red  appear  on 
the  right,  then  the  case  is  one  of  myopia.  The  further 
apart  the  images  are,  the  greater  is  the  ametropia. 


66  THE    REFRACTION    OF    THE    EYE 


CHAPTER   IV 

THE    OPHTHALMOSCOPE 

The  Ophthalmoscope  furnishes  us  with  several 
methods  for  determining  the  refraction  of  the  eyes. 

a.  The  indirect  method. 

h.  The  direct  method. 

c.  Retinoscopy. 

The  Indirect  Method. — By  the  indirect  method  we 
obtain  an  inverted  image  of  the  disc  by  means  of  a  bi- 
convex lens  placed  in  front  of  the  eye.  In  emmet ropia 
(Fig.  37)   rays  coming  from  a  emerge  from  the  eye 

Fig.  37. 


parallel,  and  are  focussed  by  the  convex  lens  at  a, 
rays  coming  from  b  are  focussed  at  h,  so  also  with 
rays  coming  from  every  part  of  a  b;  an  inverted 
image  of  A  B  is  therefore  formed  in  the  air  at  h  a,  the 
princij^tal  focus  of  the  biconvex  lens. 


THE    OPHTHALMOSCOPE  67 

In  hypermetropia  (Fig.  38)  the  rays  from  A  emerge 
Fig.  38. 


divergent^  so  also  of  course  those  from  b  ;  if  these  rays 
are  continued  backwards,  they  will  meet  behind  the 
eye  (at  the  punctum  remotum),  and  there  form  an 
enlarged  upright  image  (a  j3)  of  a  b  ;  it  is  of  this 
imaginary  projected  image  that  we  obtain  by  the  help 
of  the  biconvex  lens  a  final  inverted  image  (b  a), 
situated  in  front  of  the  lens  beyond  its  principal  focus. 

Fig.  39. 


In  myopia  (Fig.  39)  the  rays  from  A  and  b  emerge 
from  the  eye  convergent,  forming  an  inverted  aerial 
image  in  front  of  the  eye  at  j3  a,  its  punctum  remotum. 
It  is  of  this  image  we  obtain,  with  a  biconvex  lens 


68  THE  REFRACTION    OF    THE    EYE 

placed  between  it  and  the  eye,  a  final  image  {h  a) 
situated  within  the  focus  of  the  biconvex  lens. 

By  this  method  we  are  able  to  detect  the  form  of 
ametropia  by  the  changes  which  take  place  in  the  size 
and  shape  of  the  optic  disc,  always  remembering  that 
the  inverted  image  of  the  disc,  produced  by  a  convex 
lens  at  a  certain  fixed  distance  from  the  cornea,  is 
larger  in  hypermetropia,  and  smaller  in  myopia,  than 
in  emmetropia.  When  the  lens  is  held  close  to  the 
patient^s  eye,  and  then  gradually  withdrawn,  while 
the  aerial  image  of  the  disc  is  steadily  kept  in  view ; 
the  rapidity  with  which  any  increase  or  decrease 
takes  place  in  the  size  of  this  image  gives  an  indica- 
tion of  the  amount  of  the  refractive  error. 

If  no  change  take  place  in  the  size  of  the  image  on 
thus  withdrawing  the  objective  the  case  is  one  of 
emmetropia,  because  the  rays  issue  from  such  an  eye 

Fig  40. 


E.  Emmetropic  eye.  Rays  issuing  parallel,  image  formed 
at  the  principal  focus  of  the  lens,  no  matter  at  what 
distance  the  lens  is  from  the  eye. 

parallel,  and  the  image  formed  by  the  object-glass  will 
always  be  situated  at  its  principal  focus,  no  matter 
at  what  distance  the  glass  is  from  the  observed  eye 


INDIEECT    EXAMINATION  69 

(Fig.  40).  As  the  distance  of  the  image  from  the 
object-lens  is  always  the  same^  the  size  of  the  image 
will  also  be  the  same. 

If  diminution  take  place  in  the  size  of  the  image, 
the  case  is  one  of  hypermetropia,  and  the  greater  the 
diminution  the  higher  is  the  hypermetropia. 

Fig.  41. 


Lens  at  4  cm.  from  the  cornea. 
Fig.  42. 


Lens  at  12  cm.  from  the  cornea. 

H.  Hypermetropic  eye.  c.  The  centre  of  the  lens.  ab.  Image 
on  the  retina,  ah.  Projected  image.  /3  a.  The  final  image 
formed  by  the  objective. 

This  change  in  size  may  be  explained  by  remem- 
bering that  in  hypermetropia  the  image  of  the  disc 
formed  by  the    object-glass   is   situated   beyond   its 


70  THE  EEFRACTION    OF    THE    EYE 

principal  focus_,  owing  to  the  rays  issuing  from  the 
eye  being  divergent ;  the  relative  size  of  the  final 
image  j3  a  to  the  object  a  h  will  therefore  vary 
directly  as  the  length  c  a,  and  inversely  as  the  length 
C  a  so  that  on  withdrawing  the  lens  c  from  the  ob- 
served eye  c  a  diminishes  and  c  a  increases ;  there- 
fore the  ratio  oi  a  j3  to  a  h  diminishes^  i.  e.  the  size 
of  the  image  diminishes.  The  two  diagrams  Figs.  41 
and  42  show  images  formed  by  the  object-glass 
when  held  at  4  cm.  and  at  12  cm.  from  the  cornea,  the 
latter  image  being  the  smaller. 

If  the  image  become  larger  on  withdrawing  the 
object-glass,  the  case  is  one  of  myopia ;  the  greater 
the  increase  of  the  image,  the  higher  the  myopia. 

This  increase  in  the  size  of  the  image  can  also  be 
explained  with  the  help  of  mathematics,  remembering 
that  in  myopia  an  inverted  image  is  formed  in  front 
of  the  eye  (Fig.  45),  and  it  is  of  this  we  obtain  an 
image  with  a  convex  glass  placed  between  the  eye 
and  the  inverted  image,  which  we  must  regard  as  the 
object ;  the  object  and  its  image  being  both  on  the 
same  side  of  the  lens. 

In  astigmatism,  the  disc,  instead  of  appearing  round, 
is  frequently  oval.  If  the  image  of  the  disc  decrease 
in  size  in  one  meridian,  while  the  other  remain  sta- 
tionary as  the  objective  is  withdrawn  from  the  eye, 
it  is  a  case  of  simple  hypermetropic  astigmatism.  If 
the  whole  disc  decrease  in  size,  one  meridian  diminish- 
ing more  than  the  other,  it  is  compound  hypermetropic 
astigmatism,  the  meridian  being  most  hypermetropic 
which  diminishes  most. 


CONCAVE    MIREOE    AT    A  DISTANCE  71 

Increase  in  one  meridian,  tlie  other  remaining 
stationary,  indicates  simple  myopic  astigmatism. 

Increase  in  the  size  of  the  disc,  but  one  meridian 
increasing  more  than  the  other,  indicates  compound 
myopic  astigmatism,  that  meridian  being  most  myopic 
which  increases  most. 

If  one  meridian  increase  while  the  other  decrease, 
mixed  astigmatism  is  our  diagnosis. 

The  Large  Concave  Mirror  at  a  Distance. — If  the 
observer  be  able  to  see  the  disc  or  some  of  the  vessels 
with  the  mirror  alone  at  a  distance  from  the  patient, 
the  case  is  one  of  hypermetropia  or  myopia.  The 
explanation  of  this  is,  that  in  emmetropia  (Fig.  43) 
the  rays  which  come  from  the  two  extremities  of  the 
disc  (a  b)  emerge  as  two  sets  of  parallel  rays  in  the 
Fig.  43. 


same  direction  as  the  rays  A  c,  b  d,  which,  having 
passed  through  the  nodal  point,  undergo  no  refraction. 
These  two  sets  of  rays  soon  diverge,  leaving  a  space 
between  them,  so  that  an  observer  (unless  he  be  quite 
close  to  the  observed  eye)  is  able  only  to  bring  rays 
from  one  point  to  a  focus  on  his  retina;  and  there- 
fore, at  a  distance  from  the  eye,  the  observer  sees  only 
a  general  illumination. 


72  THE    EEFRACTION    OF    THE    EYE 

In  hypermetropia  (Fig.  44)  the  rays  from  the  two 
points  A  B  emerge  from  the  eye  in  two  sets  of  diverg- 
ing raySj  in  the  same  direction  as  the  rays  A  c_,  B  d, 
which  undergo  no  refraction.  These  diverging  rays 
have  the  appearance  of  coming  from  two  points  {a  h) 
behind  the  eye^  where  an  erect  imaginary  image  is 
formed. 

Fig.  44. 


The  more  the  rays  diverge  on  exit,  the  sooner  they 
will  meet  when  prolonged  backwards ;  and  hence  the 
greater  the  hypermetropia,  the  nearer  will  the  image 
be  to  the  nodal  point. 

Fig.  45. 


The  observer  at  a  distance  sees  a  clear,  erect  image 
which  is  formed  behind  the  eye. 

In  myopia  (Fig.  45)  the  rays  from  the  two  points 


DIEECT    EXAMINATION  73 

(a  b)  emerge  as  two  converging  sets  of  rays,  which 
meet  at  a  h  on  their  secondary  axes,  thus  forming  an 
inverted  image  in  front  of  the  eye.  This  image  can 
be  distinctly  seen  by  the  observer  if  he  be  at  a 
sufficient  distance  from  the  point,  and  accommodating 
for  the  particular  spot  at  which  the  aerial  image  is 
formed.  The  higher  the  myopia,  the  nearer  to  the 
eye  will  this  image  be  formed. 

From  the  above  observations  it  will  be  understood 
that  if  the  observer  now  move  his  head  from  side  to 
side,  and  the  vessels  of  the  disc  are  seen  to  move  in 
the  same  direction,  the  case  will  be  one  of  hyperme- 
tropia,  the  image  formed  being  an  erect  one. 

Should  the  vessels  move  in  the  opposite  direction 
to  the  observer's  head  the  case  will  be  one  of  myopia, 
the  image  being  an  inverted  one  formed  in  the  air  in 
front  of  the  eye. 

If  the  vessels  of  only  one  meridian  are  visible,  then 
we  have  a  case  of  astigmatism,  hypermetropic  if 
moving  in  the  same,  and  myopic  if  moving  in  the 
opposite  direction  to  the  observer's  head,  that 
meridian  being  ametropic  which  is  at  right  angles  to 
the  vessels  seen. 

In  mixed  astigmatism  the  vessels  of  one  meridian 
move  against  the  observer's  movements,  and  those  of 
the  other  meridian  with  them :  this  is  difficult  to  see. 

Thus  we  have  obtained  an  indication  of  the  form  of 
ametropia.  We  may,  however,  estimate  the  amount 
of  error  by  means  of  a  refracting  ophthalmoscope,  of 
which  there  are  many. 

The  Direct  Method. — By  the  direct  examination  we 


74  THE    REFRACTION    OP    THE    EYE 

obtain  mucli  more  important  information,  not  only  of 
a  qualitative,  but  also  of  a  quantitative  character. 

In  endeavouring  thus  to  estimate  the  refraction,  it 
is  essential  that  the  accommodation  of  both  the  patient 
and  observer  be  suspended.  The  observer  first  cor- 
rects any  ametropia  that  he  may  have,  either  by 
having  the  proper  correction  in  a  suitable  clip  behind 
the  sight-hole  of  his  ophthalmoscope,  or  he  may  deduct 
his  own  ametropia  from  the  glass  which  corrects  the 
refraction  of  the  patient  and  himself  in  the  manner  to 
be  presently  described.  He  then  sits  or  stands  as  he 
may  prefer  on  the  same  side  as  the  eye  he  is  about  to 
examine,  so  as  to  use  his  right  eye  for  the  patient^s 
right,  and  his  left  for  the  patient^s  left. 

The  light  is  placed  on  the  side  to  be  examined,  a 
little  behind  and  on  a  level  with  the  patient's  ear ; 
then  with  the  mirror  held  close  to  the  eye  to  be 
examined,  so  that  the  ophthalmoscope  may  occupy  as 
nearly  as  possible  the  position  of  the  spectacle  glass, 
the  observer  looks  for  the  disc.  We  really  wish  to 
estimate  the  refraction  at  the  macula,  but  to  this  there 
are  several  obstacles :  the  light  falling  on  this,  the 
most  sensitive  part  of  the  retina,  has  a  very  dazzling 
unpleasant  effect  for  the  patient,  and  causes  the  pupil 
to  contract  vigorously,  the  reflex  from  the  cornea  and 
the  lens  is  exactly  in  the  line  of  vision,  and  further 
there  are  no  convenient  vessels  in  this  part  which  we 
may  fix  as  test  objects ;  whereas  the  disc  is  but  little 
sensible  to  light,  and  the  vessels  of  this  part,  as  well 
as  the  margins  of  the  disc  itself,  are  very  convenient 
for  our  purpose,  and  although  occasionally  the  refrac- 


DIRECT    EXAMINATION  75 

tion  of  the  macula  and  disc  are  not  exactly  the  same, 
practically  it  is  sufficiently  accurate  to  take  that  of 
the  latter. 

To  estimate  the  refraction  by  the  direct  method, 
it  is  necessary  that  the  patient's  accommodation 
should  be  relaxed;  this  will  generally  be  the  case 
when  the  examination  is  made  in  a  dark  room ;  or  a 
mydriatic  may  be  used ;  then,  if  the  observer's  own 
accommodation  be  suspended,  and  the  image  of  the 
disc  appear  quite  clear  and  distinct,  the  case  is  one  of 
emmetropia.  This  we  know,-  because  rays  coming 
from  an  emmetropic  eye  (Fig.  46,  e)  in  a  state  of  repose 
Fig.  46. 


will  issue  parallel,  and  the  observing  eye  receiving 
these  rays  will,  if  emmetropic  with  its  accommodation 
suspended  (which  often  requires  great  practice),  be 
adapted  for  parallel  rays,  so  that  a  clear  image  of  a 
in  the  observed  eye  will  be  formed  at  h  on  the  retina 
of  the  observing  eye. 

Supposing  the  image  does  not  appear  clear  and  dis- 
tinct without  an  effort  of  accommodation,  then  we  turn 
the  wheel  of  the  ophthalmoscope  so  as  to  bring  for- 
ward convex  glasses  in  front  of  the  observing  eye. 
The  strongest  positive  glass  with  which  we  are  able  to 


76  THE    REFRACTION    OF    THE    EYE 

get  a  perfectly  clear  image  of  the  disc  is  a  measure 
of  the  hypermetropia,  because  rays  coming  from  a 
(Fig.  47)  in  the  hypermetropic  eye  (h)  issue  in  a 
divergent  direction  as  though  coming  from  R^  the 
punctum  remotum  behind  the  eye.  The  convex  lens 
L  renders  these  rays  parallel,  and  they  then  focus  at  h, 
on  the  retina  of  the  observing  emmetropic  eye  (e). 

Fig.  47. 


In  practice  many  observers  find  it  difficult  or  im- 
possible to  tell  if  their  own  accommodation  be  com- 
pletely relaxed,  so  that  if  they  see  clearly  the  disc  of 
the  patient  under  examination,  they  do  not  at  once 
assume  that  he  is  emmetropic,  but  only  do  so  on  find- 
ing that  the  weakest  convex  glass  behind  the  ophthal- 
moscope impairs  the  clearness  of  the  image. 

If,  however,  the  image  of  the  disc  appear  indistinct, 
and  the  convex  glass,  instead  of  rendering  the  image 
clearer,  have  the  opposite  effect,  we  must  turn  the 
wheel  of  the  ophthalmoscope  in  the  other  direction, 
and  so  bring  forward  the  concave  glasses.  The  weakest 
with  which  the  details  of  the  fundus  can  be  clearly  seen 
is  a  measure  of  the  myopia,  because  any  stronger  glass 
merely  brings  into  play  the  accommodation  of  the 
observer.     Rays  from  a  (Fig.  48)   leave  the  myopic 


DIRECT    EXAMINATION  77 

eye  (m)  so  convergent,  that  they  would  meet  at  (r) 

the  punctum  remotum.     The  concave  lens  l  renders 

these  rays  parallel  before  falling  on  the  relaxed  eye 

(e)  of  the  observer. 

Fig.  48. 


^^ 


If  the  ophthalmoscope  is  not  held  very  close  to  the 
eye,  we  must  deduct  from  the  focal  distance  of  the 
lens  the  distance  between  the  cornea  and  the  instru- 
ment in  hypermetropia,  adding  them  together  in  myopia 
(p.  118). 

If  astigmatism  exist,  the  plan  is  to  find  the  glass 
which  enables  the  vertical  vessels  and  lateral  sides  of 
the  disc  to  be  seen  distinctly,  and  then  the  glass  with 
which  the  vessels  at  right  angles  are  best  seen. 

Suppose  the  vertical  vessels  and  lateral  sides  of  the 
disc  appear  distinct  without  any  glass,  then  the  hori- 
zontal meridian,  i.  e.  the  meridian  at  right  angles  to 
the  vessels  clearly  seen,  is  emmetropic ;  and  suppose 
also  that  the  horizontal  vessels  with  the  upper  and 
lower  borders  of  the  disc  require  a  convex  or  con- 
cave glass  to  render  them  clear  and  distinct,  then  the 
vertical  meridian  is  hypermetropic  or  myopic,  and 
the  case  is  one  of  simple  hypermetropic  or  myopic 
astigmatism. 

If  both  the  vertical  and  horizontal  vessels  can  be 


78  THE    EEFRACTION    OP    THE    EYE 

seen  with  convex  glasses,  a  stronger  one  being  required 
for  the  vertical  than  for  the  horizontal  vessels_,  then  the 
case  is  one  of  compound  hypermetropic  astigmatism, 
the  horizontal  meridian  being  the  more  hypermetropic. 

If  both  meridians  had  required  concave  glasses,  but 
of  different  strengths,  then  the  case  would  be  one  of 
compound  myopic  astigmatism. 

If  the  vertical  vessels  and  the  lateral  sides  of  the 
disc  can  be  seen  distinctly  through  a  convex  glass, 
while  the  horizontal  vessels  require  a  concave  glass, 
the  case  is  one  of  mixed  astigmatism,  the  horizontal 
meridian  being  hypermetropic,  the  vertical  meridian 
myopic. 

The  essential  point  to  remember  is,  that  the  glass 
with  which  the  vessels  in  one  direction  are  seen  is  a 
measure  of  the  refraction  of  the  meridian  at  right 
angles  to  these  vessels. 

The  estimation  of  the  refraction  by  the  direct  oph- 
thalmoscopic method  is  exceedingly  valuable,  but 
requires  great  practice;  some  observers  find  con- 
siderable difficulty  in  relaxing  their  accommodation 
completely,  even  after  long  practice. 

The  student  should  take  every  opportunity  to 
become  thoroughly  proficient  in  estimating  the  re- 
fraction by  this  method;  in  fact,  every  case  that  is 
not  of  an  inflammatory  character  should  be  examined 
with  the  ophthalmoscope,  and  the  refraction  as  esti- 
mated by  the  direct  method  recorded  as  a  matter  of 
routine ;  the  ophthalmoscopic  examination  may  con- 
veniently follow  the  testing  of  the  patient's  visual 
acuity. 


DIRECT    EXAMINATION  79 

In  hypermetropia  and  myopia  one  is  able  to  esti- 
mate the  amount  of  error  accurately,  and  in  cases 
of  astigmatism  where  the  chief  meridians  are  hori- 
zontal and  vertical  one  can  come  very  near  the  exact 
correction,  and  without  necessarily  subjecting  the 
patient  to  the  inconvenience  of  a  mydriatic :  when 
the  meridians  are  oblique  the  estimation  is  more 
difficult,  because  we  may  find  no  vessel  whose  course 
exactly  corresponds  with  the  chief  meridians.  Still 
the  more  this  method  is  practised  the  more  accurate 
will  be  the  results  obtained.  The  correction  must 
always  be  confirmed  by  trying  the  patient  at  the  test 
types  with  lenses,  making  any  slight  alteration  that 
may  be  necessary. 

It  is  also  an  additional  advantage  that  one  can  esti- 
mate the  refraction  at  the  same  time  that  one  makes 
an  examination  of  the  fundus. 

The  comparison  of  the  direct  and  indirect  methods 
of  examination  is  very  useful  in  astigmatism.  If,  for 
instance,  the  disc  is  elongated  vertically  in  the  erect, 
and  oval  horizontally  in  the  inverted  image,  we  know 
that  the  curvature  of  the  cornea  is  greater  in  the 
vertical  than  in  the  horizontal  meridian  (see  Figs.  86 
and  87). 

The  ametropic  observer  must  always  remember, 
when  using  the  direct  method  for  the  estimation  of 
errors  of  refraction,  that  he  must  correct  his  own 
defect  either  by  wearing  spectacles  or  by  having  a 
suitable  glass  in  a  clip  behind  his  opthalmoscope ; 
he  is  then  in  the  position  of  an  emmetrope ;  but,  if 
he  prefer  it,  he  may  subtract  the  amount  of  his  own 


80 


THE    REFRACTION    OF    THE    EYE 


hyper  me  tropia  or  myopia  from  the  glass  with  which 
he  sees  clearly  the  patient's  discs.  Thus,  if  the  ob- 
server have  2  D.  of  hypermetropia^  and  require  +  3  D. 
to  see  the  fundus  clearly  (  +  3  D.)  -  ( +  2  D.)  =  +  1  D., 
the  patient  would  have  1  D.  of  hypermetropia ;  had 
he  required  —  2  D.,  then  the  patient  would  have  4  D. 
of  myopia,  because  (—2  D.)  —  (+  2  D.)  =  —  4  D. 

The  same  with  the  myopic  observer ;  if  his  myopia 
amount  to  3  D._,  then  he  will  require  —  3  D.  to  see 
clearly  the  emmetropic  fundus ;  if  he  see  the  disc  well 
without  a  glass,  then  the  eye  under  examination  has 
3  D.  of  hypermetropia;  if  he  require  a  +  2  D.,  then 
the  hypermetropia  will  be  5  D._,  and  so  on. 

Ametropia  may  also  be  easily  recognised  in  the 
following  manner  : — The  fundus  being  illuminated  by 
a  mirror  about  one  metre  from  the  patient,  if  the  eye 
be  emmetropic  the  rays  of  light  will  return  parallel  to 
one  another,  and  a  red  reflex  can  only  be  obtained  when 
the  observing  eye  is  in  the  path  of  these  rays,  that  is 
behind  the  perforation  of  the  mirror.  If  hyperme- 
tropic the  returning  rays  will  diverge  (Fig.  49),  and 

Fig.  49. 


the  observer  will  notice,  as  he  moves  his  eye  (b)  from 
behind  the  mirror  at  l    (and   at  right  angles  to  the 


WITH    THE    MIREOR 


81 


visual  axis  of  the  patient,  who  should  fix  on  the  centre 
of  the  mirror),  that  the  last  ray  of  light  {a  h')  is  seen, 
or,  in  other  words,  the  red  reflex  disappears,  on  the 
same  side  of  the  pupil  as  that  to  which  the  observer 
moves  his  head. 

If    the    eye    be    myopic    the    rays    will    converge, 
cross,  and  diverge  (Fig.  50)  ;  when  the  error  is   1  D. 

Fig.  50. 


or  more,  the  last  ray  of  light  is  seen,  or  the  red  reflex 
disappears,  on  the  opposite  side  of  the  pupil.  A  single 
trial  of  this  will  prove  its  correctness. 

The  endeavour  to  estimate  the  amount  of  myopia 
or  hypermetropia  by  measuring  the  distance  betAveen 
the  perforation  of  the  mirror,  and  the  point  at  which 
the  last  ray  was  seen,  has  been  unsuccessful,  owing  to 
the  varying  size  of  the  pupil. 

The  ophthalmometer  of  Javal  and  Schiotz,  and 
Tweedy's  optometer,  can,  I  think,  be  more  conveniently 
considered  when  treating  of  astigmatism. 


82  THE    REFRACTION    OF    THE    EYE 


CHAPTER  V 

RETINOSCOPY 

Retinoscopy,  or  the  shadow  test^  is  deservedly  one 
of  the  most  popular  objective  methods  of  estimating 
the  refraction  of  the  eye. 

It  has  the  great  advantage  of  being  easily  learnt, 
and  can  be  carried  out  quickly,  saving  much  time  in 
difficult  cases  of  astigmatism ;  it  is  especially  useful 
in  young  children,  in  amblyopic  patients,  and  in 
malingerers  ;  besides,  it  enables  very  small  degrees  of 
astigmatism  to  be  detected  which  but  for  this  method 
would  probably  escape  notice.  Retinoscopy  may  be 
carried  out  either  with  a  plane  or  a  concave  mirror. 

The  plane  mirror  is  now  almost  universally  used ; 
the  shadows  are  better  seen,  and  the  results  obtained 
are  more  exact  than  with  the  concave  mirror. 

The  observer  who  has  not  good  vision,  f,  is  at  a 
disadvantage  in  employing  retinoscopy,  as  he  will  see 
the  shadows  less  clearly  ;  he  should  therefore  wear 
his  own  correcting  glasses  if  he  has  any  error  of 
refraction. 

Light. — A  good  light  for  retinoscopy  is  the  incan- 
descent focal  lamp  of  16  candle-power.  These 
lamps  are  made  with  one  side   of  ground  glass,  the 


EETINOSCOPY 


83 


other    clear,    and    the    filament    in    the    shape  of    a 
gridiron  so  as  to  give  a  greater  light  surface.     For 


Fig.  51. 


retinoscopy  the  transparent  side  of  the  lamp  is  turned 
towards  the  observer,  the  ground  side  being  re- 
served for  the  ordinary  ophthalmoscopic  examination 

Fig.  52. 


Where    the    electric    light    is    not    available    a    gas 
A  rgand  burner  or   an    oil   lamp    may  be   used  :   the 


84  THE    REFRACTION    OP    THE    EYE 

flame  should  be  of  fair  size  with  well-defined  edges. 
The  light  should  be  on  a  bracket  which  can  be  moved 
in  any  direction. 

The  light  is  usually  placed  over  the  patient's  head 
and  slightly  behind,  so  as  to  leave  the  eyes  in  shade 
(Fig.  51).  Some  observers  prefer  a  bright  screened 
light,  coming  from  a  diaphragm  opening  of  5  to  10  mm., 
placed  2  cm.  from  the  mirror  (Fig.  52)  ;  the  illumina- 
tion is  brighter  the  nearer  the  light  is  to  the  mirror. 

A  dark  room  is  an  advantage,  otherwise  the 
curtains  of  the  room  must  be  drawn ;  for  the  darker 
the  room  the  easier  is  retinoscopy. 

Retinoscopy  with  the  Plane  Mirror. 

Most  modern  ophthalmoscopes  are  provided  with  a 
suitable  plane  mirror.     The   drawing  below  shows  a 
simple  and  convenient  one  for  the  pocket. 
Fig.  53. 


If  light  from  the  ophthalmoscope  lamp  be  reflected 
into  the  eye  by  means  of   a  plane  mirror,  at  a  dis- 


RETINOSCOPY  85 

tance  of  a  metre  or  so,  an  observer  looking  througli 
the  sight-hole  of  the  mirror  will  notice  the  ordinary 
red  fundus  reflex ;  on  slightly  rotating  the  mirror,  the 
illuminated  area  of  the  pupil  may  disappear  (or,  what 
may  be  more  easily  seen,  the  edge  of  the  shadow 
bounding  this  illuminated  area  may  appear),  on  the 
same  side  as  the  rotation  or  in  the  opposite  direction, 
according  to  the  refraction  of  the  eye  under  observa- 
tion :  thus  if  the  mirror  be  rotated  to  the  right,  and 
the  edge  of  the  shadow  move  across  the  pupil  also  to 
the  right,  i.  e.  in  the  same  direction  as  the  rotation  of 

Fig.  54. 


the  mirror,  the  case  is  one  of  hypermetropia ;  whereas 
if  the  shadow  had  moved  in  the  opposite  direction  to 
the  mirror,  the  case  would  be  one  of  myopia. 

This  method  of  employing  retinoscopy  is  so  simple 
that  a  few  practical  trials  will  suffice  to  make  it 
understood,  although,  of  course,  as  in  all  other  mani- 
pulations, some  little  practice  is  required  in  giving  to 
the  mirror  the  necessary  movements,  and  in  enabling 
one  to  appreciate  what  is  seen. 

The  illumination  and  shadows  we  see  are  an  image 
of  the  lamp  with  the  surrounding  shadow  formed  on 
the  retina  of  the  observed  eye. 


86  THE    REFRACTION    OF    THE    EYE 

When  diverging  rays  of  light  fall  on  a  plane 
mirror  they  are  reflected  as  divergent  rays,  as  if 
coming  from  a  point  behind  the  mirror ;  so  that  the 
image  of  the  plane  mirror  is  a  virtual  one  :  the 
diverging  rays  passing  through  the  pupil  of  the  eye 
under  observation  are  brought  more  or  less  to  a  focus 
according  to  the  refractive  condition  of  the  eye ;  on 
rotating  the  mirror  in  one  direction  the  retinal  image 
will  move  in  the  same  direction.  The  following 
diagram  will  help  to  make  this  clear. 
Fig.  55. 


Rays  of  light  from  l  fall  on  the  plane  mirror  A,  and 
are  reflected  as  divergent  rays  into  the  eye  as  if 
coming  from  point  G  behind  the  mirror ;  these  rays 
focus  on  the  retina  at  c.  On  tilting  the  mirror  into 
position  B,  the  rays  from  l  diverge  from  the  mirror 
(as  if  coming  from  h)  and  focus  at  d  ;  therefore  the 
real  movement  of  the  light  on  the  retina  is  with  the 
mirror.  Hence  the  movement  of  the  retinal  image 
is  always  with  the  mirror ;   but  as  these  movements 


RETINOSCOPY  87 

are  seen  tlirongli  the  dioptri3  system  of  the  eye^  and 
thereby  undergo  refraction^  the  apparent  may  differ 
from  the  real  movements. 

The  retinal  image  the  observer  sees  of  the  lamp 
and  its  surrounding  shadow  are  formed  in  the  same 
manner  as  all  other  images. 

In  hypermetropia  the  final  image  of  the  lamp  and 
its  surrounding  shadow,  produced  by  the  plane 
mirror,  is  an  erect  one  formed  behind  the  eye,  and 
as  it  is  viewed  through  the  dioptric  system  of  the  eye, 
it  therefore  moves  with  the  mirror  (Fig.  44). 

In  myopia  the  final  image  is  an  inverted  one,  pro- 
jected forwards.  This,  therefore,  moves  against  the 
mirror  (Fig.  45). 

If,  however,  the  observer  be  nearer  than  the 
patient's  far  point  the  image  will  move  with  the 
mirror.  This  is  the  case  in  low  degrees  of  myopia, 
where  the  patient's  far  point  is  beyond  120  cm. 

Therefore,  if  the  image  move  against  the  mirror, 
the  case  is  certainly  one  of  myopia.  If  it  move  with 
the  mirror,  it  is  most  likely  one  of  hypermetropia ; 
but  it  may  be  emmetropia,  or  a  very  low  degree  of 
myopia  (  —  '50  D.). 

The  movements  tell  us  the  form  of  ametropia  we 
have  to  deal  with.  The  extent  of  the  movements  on 
rotation  of  the  mirror,  the  clearness  of  the  image, 
and  the  brightness  of  its  edge,  enable  us  to  judge 
approximately  the  amount  of  ametropia  to  be  cor- 
rected; some  practice,  however,  is  required  before 
we  can  form  an  opinion  with  anything  like  accuracy. 

The    extent   and   rate   of   movement  is    always  in 


88  THE    REFRACTION    OF    THE    EYE 

inverse  proportion  to  the  ametropia ;  the  greater  the 
error  of  refraction^  the  less  the  movement,  and  the 
slower  it  takes  place.  This  may  be  explained  in  the 
following  way  : 

Suppose  A  to  be  the  image  of  a  luminous  point 
formed  on  the  retina,  and  that  a  line  be  drawn  from 
A,  through  the  nodal  point  b  to  c.  Now,  if  the  case 
be  one  of  myopia  (Fig.  56),  an  inverted  projected 
image  of  a  is  formed  somewhere  on  this  line,  say  at 
c.     The  higher  the  myopia,  the  nearer  to  the  nodal 

Fig.  56. 


point  will  this  image  be;  and  hence  in  a  higher 
degree  of  myopia  we  may  suppose  it  formed  as  near 
as  D.  If  the  mirror  be  now  rotated,  so  that  it  take  up 
the  position  of  the  dotted  line  m',  c  will  have  moved 
to  c,  and  i>  to  d ;  hence  it  is  clear  that  c  has  made  a 
greater  movement  than  d. 

Had  the  case  been  one  of  hypermetropia  (Fig.  57), 
the  image  would  have  been  projected  backwards, 
and,  as  in  myopia,  the  higher  the  degree  of  error  the 
nearer  to  the  nodal  point  is  the  image  formed. 

In  this  case  the  line  from  the  nodal  point  b  to  a  is 


EETINOSCOPY  89 

prolonged  backwards^  and  tlie  image  of  the  luminous 
point  in  a  low  degree  of  liypermetropia  is  formed  say 
at  c,  and  in  a  higher  degree  say  at  d.  On  moving 
the  mirror  into  the  position  of  the  dotted  line  m',  c 
moves  to  c,  and  d  to  cZ ;  hence  it  is  clear  that  c  has 
made  a  greater  movement  than  d. 

Fig.  57. 


Therefore,  as  the  ametropia  increases,  the  extent 
of  the  movement  of  the  image  decreases.  The  clear- 
ness of  the  image  and  the  brightness  of  its  edge 
decrease  as  the  ametropia  increases. 

Aetificial  Cycloplegia. — A  mydriatic  is  not  abso- 
lutely essential;  still,  when  we  have  to  examine  young 
people  and  children,  the  use  of  atropine  is  certainly 
advisable.  In  the  first  place,  the  dilatation  of  the 
pupil  renders  our  examination  so  much  easier;  and 
secondly,  atropine  enables  us  to  arrive  at  a  more 
accurate  estimation  by  thoroughly  paralysing  the 
accommodation :  for  although  the  examination  take 
place  in  a  dark  room,  and  with  the  patient  looking 
into  distance,  it  must  be  remembered  that  there  is 
often  (especially  in  the  case  of  children)  some  accom- 


90  THE    REFRACTION    OP    THE    EYE 

modation  still  in  force ;  or  there  may  be  spasm  of  tlie 
ciliary  muscle. 

In  persons  over  the  age  of  twenty,  atropine  is  not 
usually  employed,  owing  to  the  great  discomfort 
entailed  by  its  paralysing  effect  on  the  ciliary  muscles, 
which  lasts  for  seven  or  eight  days,  and  because  the 
use  of  this  drug  has  occasionally  produced  glaucoma 
in  people  beyond  middle  life ;  we  may,  however,  wish 
to  dilate  the  pupils  with  a  mydriatic  that  acts  fully 
and  quickly,  the  effects  of  which  last  but  a  short  time. 
The  most  convenient  combination  for  this  purpose  is — 

P>   Homitropinse  Hydrobrotnatis,  ^r.  iv ; 

Cocainse  Hydrochloratis,  gr.  x  ; 

Acidi  Salicylic!,  gr.  j ; 

Aq  Destillatse,  3j. 
Ft.  guttse. 

One  drop  of  this  solution  applied  two  or  three 
times  at  intervals  of  ten  minutes  produces  rapidly  a 
maximum  dilatation  of  the  pupil,  which  passes  off  in 
about  twelve  hours. 

Another  great  advantage  of  a  mydriatic  is  that  the 
refraction  at  the  macula  can  be  measured,  whereas 
when  the  pupil  is  not  dilated  we  have  to  be  satisfied 
with  the  refraction  at  the  optic  discs,  which  may 
occasionally  vary  considerably  from  that  found  at  the 
macula :  the  estimation  of  the  refraction  at  the  macula 
constitutes  one  of  the  chief  advantages  that  retino- 
scopy  possesses  over  the  measurement  obtained  by 
the  direct  method. 

Therefore  atropine  should  be  used  either  in  the 
form  of  drops  or  ointment — 


RETINOSCOPY  91 

1.  In  all  cases  of  concomitant  squint. 

2.  In  hypermetropes  under  twenty. 

3.  In  cases  of  defective  vision  under  eighteen,  due 
to  myopia  or  astigmatism. 

Homatropine  and  cocaine  may  be  used  witli  ad- 
vantage— 

1.  In  all  cases  of  astigmatism  over  twenty. 

2.  Where  the  correction  by  glasses  has  failed  to 
relieve  a  patient's  discomfort. 

In  all  persons  over  thirty,  care  should  be  taken  to 
estimate  the  tension  of  the  eye  before  applying  a 
mydriatic,  and  when  it  has  been  applied  a  drop  of  a 
solution  of  eserine  (gr.  j  to  5j)  should  be  instilled 
into  the  eye  after  the  examination  is  completed. 

In  patients  above  the  age  of  twenty-five  a  mydriatic 
is  not  always  necessary ;  many  simple  cases  of  astig- 
matism can  be  worked  out  rapidly  and  accurately 
without  one. 

Remember  the  patient  must  not  look  directly  at  the 
mirror,  but  slightly  inwards  when  using  retinoscopy 
without  a  mydriatic. 

To  proceed  now  to  the  estimation  of  the  refraction 
by  retinoscopy. 

The  patient  being  seated  in  the  dark  room,  the 
pupils  dilated,  and  the  lamp  over  his  head,  we  place 
a  pair  of  trial  frames  on  his  face  and  take  up  our 
position  120  cm.  or  further  (I  usually  sit  150  cm.  from 
the  patient)  in  front,  with  a  plane  mirror.  The 
patient  is  then  directed  to  look  at  the  centre  of  the 
mirror,  so  that  the  light  from  the  lamp  may  be 
reflected  along  the  visual  axis  of  the  right  eye.     On 


92  THK    EEFEACTION    OF    THE    EYE 

looking  through  the  perforation  of  the  mirror  we  get 
the  ordinary  fundus  reflex,  bright  if  the  patient  be 
emmetropic,  less  so  if  he  be  ametropia;  and  the 
greater  the  ametropia,  the  less  bright  will  the  fundus 
reflex  be.  We  now  rotate  the  mirror  on  its  vertical 
axis  to  the  right;  if  a  vertical  shadow  come  across 
the  pupil  from  the  patient^s  right,  i.  e.  in  the  same 
direction  as  the  movement  of  the  mirror,  or,  what  is 
the  same  thing,  if  the  shadow  move  in  the  same  direc- 
tion as  the  circle  of  light  on  the  patient^s  face,  the 
case  is  one  of  hypermetropia.  Should  the  edge  of 
the  image  appear  well  defined  and  move  quickly,  in 
addition  to  a  bright  fundus  reflex,  we  infer  that  the 
hypermetropia  is  of  low  degree,  and  proceed  to  cor- 
rect it.  First  place  a  weak  convex  glass,  say  +  *50  D., 
before  the  eye  in  the  spectacle  frame ;  if  the  shadow 
still  move  with  the  mirror  we  change  the  glass  for  + 
1  D.,  then  -|-  1'5  D.,  and  so  on,  until  we  find  the  glass 
with  which  no  distinct  shadow  can  be  seen. 

Supposing  this  to  be  +  2  D.,  and  that  on  trying 
+  2'5  D.  the  shadow  move  against  the  mirror,  -f  2  D. 
is  put  down  as  the  correcting  glass. 

Had  we  obtained  a  reverse  shadow  concave  glasses 
would  be  employed,  proceeding  exactly  as  before, 
commencing  with  a  weak  concave  glass,  putting  up 
stronger  and  stronger  concave  lenses  until  we  have 
neutralised  the  shadow.  This  is  put  down  as  the 
correcting  glass. 

These  glasses  are  not  the  exact  estimate  of  the 
refraction,  because  the  observer  is  not  sitting  at 
infinity,  but  at  120  cm.  from  the  eye,  so  that  when 


EETINOSCOPY  93 

no  shadow  is  obtained  we  are  sitting  practically  at 
the  patient's  far  point. 

Therefore  the  hypermetropia  is  over-corrected  1  D. 
and  the  myopia  is  under-estimated  ID.;  so  in  hyper- 
metropia we  deduct  1  D.  and  in  myopia  we  add  1  D. 
to  the  correcting  glass. 

To  sum  up^  if  the  shadow  move  with  the  mirror^  it 
may  be  weak  myopia  if  +  '50  D.  obliterates  the 
shadow;  emmetropia  if  +  1  D.  neutralise  it;  hyper- 
metropia if  a  stronger  glass  is  required  :  when  the 
shadow  moves  against  the  mirror  it  is  a  case  of 
myopia. 

In  high  myopia  a  strong  concave  glass  has  to  be 
used  for  the  correction ;  the  light  from  the  mirror  is 
so  spread  out  in  passing  through  this  lens  that  fewer 
rays  pass  into  the  eye^  therefore  the  illumination  is 
not  so  good  as  in  other  states  of  refraction,  and  the 
examination  becomes  more  difficult. 

The  points  to  be  observed  are —  (1)  the  direction  of 
the  movement  of  the  image,  as  indicating  the  kind  of 
ametropia :  (2)  the  rate  and  amount  of  movement,  (3) 
the  brightness  of  the  edge  of  the  image,  and  (4)  the 
amount  of  fundus  rerflex  all  indicate  the  degree  of 
ametropia. 

We  have  taken  notice  only  of  the  horizontal  axis, 
but  any  other  meridian  will,  of  course,  do  equally 
well,  if  the  case  be  one  of  hj^permetropia  or  myopia 
simply.  If,  however,  the  case  be  one  of  astigmatism, 
then  the  refraction  of  the  two  chief  meridians  will  differ. 

In  astigmatism,  the  image  of  the  flame  of  the  lamp 
formed  on  the  retina  is  distorted  so  as  to  be  more  or 


94  THE    EEFKACTION    OF    THE    EYE 

less  of  an  oval  form^  according  to  the  position  of  the 
retina  and  the  maximum  and  minimum  curvatures  of 
the  cornea  (Fig.  76). 

So  that^  in  astigmatism,  the  image  on  the  retina 
may  be  more  or  less  of  an  oval,  instead  of  being  either 
a  small  well-defined  image  of  the  lamp,  or  a  circle  of 
diffusion  as  in  the  case  of  emmetropia,  myopia,  or 
hypermetropia.  This  oval  may  have  its  edges  hori- 
zontal and  vertical ;  frequently,  however,  they  are 
more  or  less  oblique,  as  shown  in  the  following 
figures  (Fig.  58). 

Fig.  58. 


Oblique  shadows  in  astigmatism. 

The  oblique  movements  of  the  shadows  are  inde- 
pendent of  the  direction  in  which  the  mirror  is 
rotated. 

This  obliquity  is  produced  thus  (Fig.  59)  : — If  we 
cut  a  circular  opening  in  a  piece  of  cardboard  to 
represent  the  pupil,  and  then  place  behind  it  an  oval 
piece  of  card  which  is  to  represent  the  shadow,  so  that 
that  part  of  its  edge  which  occupies  the  pupil  has  an 
oblique  position,  then  on  moving  the  card  across  in 
the  direction  o  d,  it  has  the  appearance  of  moving  in 
the  direction  o  c,  at  right  ai^gles  to  the  edge  of  the 
card.     Hence  the  direction  of  the  shadow's  movement 


EETINOSCOPY 


95 


is  deceiving,  and  the  oblique  edge  is  due  to  the  fact 
that  only  that  edge  which  coincides  in  direction  with 
one  of  the  principal  meridians  is  seen  well  defined  by 
the  observer.  Therefore  the  apparent  movenients 
correspond  with  the  edge  of  the  shadow. 

Fig.  59. 


The  same  takes  place  in  astigmatism,  the  two  chief 
meridians  of  which  are  parallel  and  perpendicular  to 
the  edge  of  the  shadow.  In  retinoscopy,  therefore, 
when  the  edge  of  the  image  is  oblique,  we  know  at 
once  that  the  case  is  one  of  astigmatism. 

Another  characteristic  appearance  that  will  be  some- 
times met  with  in  astigmatism  is,  that  the  fundus 
illumination  may  assume  a  band-like  shape  something 
Fig.  60.  Fig.  61. 


like  Fig.  60;    and   on   tilting  the  mirror  on  an  axis 
parallel  to  this  band,  a  dark  shadow  will  appear  to 


96  THE    REFRACTION    OF    THE    EYE 

come  from  both  edges  of  the  pupil  at  once,  uniting  in 
the  centre  to  form  a  black  band_,  leaving  the  upper 
and  lower  part  of  the  fundus  illuminated  as  shown  in 
Fig.  61.  This  band  is  parallel  with  one  of  the  chief 
meridians,  and  indicates  the  point  of  exact  neutralisa- 
tion of  the  meridian  at  right  angles  to  it.  The  effect 
is  due  to  the  retinal  images  being  now  in  the  form  of 
a  line  (see  Figs.  76  and  77,  ii  and  vi). 

This  variety  of  movement  of  the  shadow  is  some- 
times spoken  of  as  "the  scissor  movement.^^ 

Supposing  we  take  a  case  in  which  the  meridians 
are  horizontal  and  vertical,  we  judge  if  one  shadow 
be  more  distinct  or  quicker  in  its  movements  than  the 
other,  though  it  is  not  always  easy  to  recognise  the 
presence  of  astigmatism  at  once,  so  that  it  may  be 
necessary  to  correct  one  meridian  before  we  can  be 
certain.  If  the  shadow  move  with  the  mirror  in 
all  meridians,  we  first  take  notice  of  the  vertical  one, 
and  put  up  in  front  of  the  patient,  in  the  spectacle 
frame,  convex  spherical  glasses,  until  the  glass  which 
neutralises  the  shadow  has  been  found.  This  is 
put  down  as  the  correcting  glass  for  the  vertical 
meridian;  let  us  suppose  that  glass  to  be  +  2  D. 
We  next  take  notice  of  the  horizontal  meridian,  and 
if  +  2  D.  is  also  the  glass  which  neutralises  the 
shadow,  then  of  course  we  know  ^the  case  is  one  of 
simple  hypermetropia.  But  supposing  the  convex 
glass  necessary  to  correct  this  meridian  is  +  4  D., 
we  indicate  it  conveniently  thus  : 

+  2  D. 

[ 

--r  4D. 


EETINOSCOPY  97 

The  case  is  one  of  compound  hypermetropic  astigma- 
tism, and  will  require  for  its  correction  +  2  D.  sphere 
combined  with  +  2  D.  cylinder  axis  vertical. 

We  will  take  another  case — that  in  which  the  vertical 
shadow  is  neutralised  by  a  +  2  J).,  while  for  the 
horizontal  shadow  —  2  D.  is  required. 

2D. 

— +  2D. 


Here  we  have  a  case  of  mixed  astigmatism,  which 
can  be  corrected  in  either  of  the  three  following 
ways  : 

1st.  —  2  D.  cylinder  axis  horizontal  combined  with 
+  2  D.  cylinder  axis  vertical ;  this  is  a  plan  seldom 
used,  and  is  not  so  easy  to  work  with  as  a  sphere  and 
a  cylinder. 

2nd.  —2D.  sphere  combined  with  -h  4  D.  cylinder 
axis  vertical,  or 

3rd.  +  2  D.  sphere  combined  with  —  4  D.  cylinder 
axis  horizontal.     The  last  is  the  preferable  plan. 

Supposing  the  axis  of  the  shadow  to  be  oblique 
(Fig.  58),  we  know  at  once  that  astigmatism  exists, 
and  we  proceed  to  correct  each  meridian  separately, 
moving  the  mirror  at  right  angles  to  the  edge  of  the 
shadow,  not  horizontally  and  vertically.  We  judge 
of  the  amount  of  obliquity  by  the  eye,  and  can  fre- 
quently tell  within  a  few  degrees.  If  the  vertical 
meridian  be  20°  out,  and  require  for  its  correction 
—  2  D.,  and  the  axis  at  right  angles  to  this  (which  will 
therefore  be  at  110°)  require  —  3  D.,  we  express  it  as 

7 


98  THE    REFRACTION    OF    THE    EYE 

in  Fig.  62,  and  correct  it  with  sphere  —  2D.  com- 
bined with  cylinder  —ID.  axis  20°  :  this  is  a  case 
of  compound  myopic  astigmatism. 

One  is  often  able  to  put  up  the  cylinder  in  the 
spectacle  frame  with  the  exact  degree   of   obliquity. 

Having  found  the  glasses  which  correct  the  two 
meridians_,  we  put  up  the  combination  in  a  spectacle 
trial  frame,  and  if  we  now  get  no  shadow  the  glasses 
are  assumed  to  be  the  right  ones,  and  we  proceed  to 
confirm  it  by  trying  the  patient  at  the  distant  type, 
making  any  slight  alteration  that  may  be  necessary. 

When  employing  retinoscopy  the  sectors  of  the 
lens   often  show  up  very  plainly  (when  no   opacity  is 

Fig.  62. 


to  be  seen) ;  this  may  be  the  earliest  sign  of  a  patho- 
logical process  taking  place  in  the  lens. 

In  irregular  astigmatism  retinoscopy  does  not  give 
satisfactory  results,  the  shadows  being  indefinite  and 
irregular.  In  conical  cornea  the  shadow  may  appear 
circular,  occupying  an  intermediate  position  between 
the  centre  and  the  edge  of  the  cornea ;  on  tilting  the 
mirror  the  shadow  appears  to  run  round  the  base  of 
the  cone  in  a  circular  direction. 


RETINOSCOPY  99 

A  further  modification  of  retinoscopy  wliicli  may 
sometimes  be  useful  is  that  proposed  by  Dr.  Jackson, 
of  Philadelphia.  The  object  is  to  find  out  the  point  of 
reversal  of  the  image.  Thus,  if  a  patient  be  a  myope 
of  2  D.,  the  observer,  at  a  distance  of  a  metre,  sees 
the  shadow  moving  against  the  mirror ;  on  coming 
near  he  will  find  the  image  disappear  at  50  cm.,  and 
on  coming  still  closer  the  image  will  move  with  the 
mirror;  the  point  of  reversal  is  therefore  at  50  cm. 
By  dividing  the  distance  of  the  point  of  reversal 
into  100  cm.  we  arrive  at  the  patient's  error.  If  the 
point  of  reversal  is  at  25  cm.,  then  the  myopia  will 
be  4  D. 

In  order  to  use  this  method  satisfactorily,  one  or 
two  points  require  attention. 

In  Simple  Myopia. — When  the  observer's  eye  has 
come  quite  close  to  the  patient's,  say  one  eighth  of  a 
metre,  and  the  inverted  image  is  still  seen,  it  is  best 
to  place  a  concave  lens  (—  8  D.)  before  the  patient's 
eye  and  then  estimate  the  amount  of  myopia  uncor- 
rected; by  adding  this  to  the  amount  which  the  lens 
used  has  corrected  we  determine  the  total  myopia 
present.  It  is  evident  that  if  the  point  of  reversal  is 
close  to  the  eye  the  error  of  a  few  centimetres  as  to 
its  position  entails  an  error  of  some  dioptres  in  the 
amount  of  myopia  present.  Therefore  we  place 
before  the  patient^s  eye  a  concave  glass  strong 
enough  to  remove  the  point  of  reversal  a  metre  or  so 
from  the  eye. 

In  Hypermetropia. — Place  before  the  patient's  eye 
a    convex    glass    strong    enough   to    over-correct  the 


100  THE    EEFEACTION    OF    THE    EYE 

hypermetropia.  Then  by  the  method  given  above, 
determine  the  degree  of  myopia  so  produced.  Deduct 
this  amount  of  myopia  from  the  strength  of  the 
convex  glass  used ;  this  will  give  the  amount  of  hyper- 
metropia  present.  Suppose,  for  example,  the  hyper- 
metropia  amounts  to  four  dioptres :  place  before  the 
eye  +  5  D.,  it  is  found  that  one  dioptre  of  myopia  is 
produced;  the  point  of  reversal  being  at  one  metre. 
Therefore  to  estimate  hypermetropia  by  this  method 
a  convex  lens  must  always  be  used. 

Emmetropia. — A  weak  convex  lens  being  placed 
before  the  eye,  the  point  of  reversal  will  be  found  to 
equal  the  strength   of  the  lens  used. 

Regular  Astigmatism. — We  find  the  point  of  rever- 
sal for  each  of  the  principal  meridians. 

Retinoscopy  with  the  Concave  Mieeor. 

When  the  concave  mirror  is  used,  then  the  move- 
ments of  the  shadows  are  in   the  opposite   direction 
Fig.  63. 


to  those   obtained  with  the  plane  mirror.     Divergiug 
rays  of  light  falling  on  the  concave  mirror  converge 


RETINOSCOPY  101 

and  form  an  inverted  image  of  tlie  lamp  between  the 
observer  and  tbe  patient^  and  this  image  becomes  the 
object.  From  this  image  rays  diverge.  Some  of 
them  enter  the  pupil  of  the  observed  eye,  and  are 
brought  more  or  less  to  a  focus  on  the  retina,  accord- 
ing to  the  refraction  of  the  eye. 

Fig.  64. 


Hence,  with  the  concave  mirror,  the  image  suffers 
another  inversion,  therefore  the  image  on  the  retina 
always  moves  against  the  movement  of  the  mirror, 
which  is  the  reverse  of  that  obtained  Avith  the  plane 


mirror. 


Fig. 


A  few  cases  from  my  note-book  will  do  more  than 
any  description  to  elucidate  the  subject  of  retino- 
scopy  with  the  plane  mirror. 


102  THE    EEFRACTION    OF    THE    EYE 

Case  1.  Spasm  of  the  Ciliary  Muscle. — Boy,  get.  10 
years,  is  brought  because  lie  is  unable  to  see  the 
bla-  J^?ard  at  school. 

B.V,-i%-lD.  =  f. 
L.V.-ia-lD.  =  f. 

Retmoscopy. — Bright  fundus  reflex,  shadows  move 
against  the  mirror.  Direct  method,  the  discs  clearly 
seen  without  a  glass. 

Guttse  atropise  sulphatis,  gr.  iv  to  5j- 

Ter  in  die. 

On  the  third  day  the  pupils  are  found  well  dilated. 

Eetinoscopy  gave  +  3  D.  in  both  eyes. 

On  trying  the  patient  with  the  test-type, 

E.V.  +  2D.=f. 

L.V.  +  2  D.  =  J. 
o 

This,  therefore,  was  a  case  of  hypermetropia  with 
spasm  of  the  ciliary  muscle  simulating  myopia.  Such 
cases  are  very  common,  and  one  should  always  be  on 
the  look-out  for  them. 

Case  2.  Hypermetropia. — A  young  woman  a3t.  15, 
suffering  from  blepharitis. 

R.V.  I  Hm.  1  D.  =  f. 


L.V.  ^  Hm.  1  D. 


6  6 

Guttse  atropise  sulphatis,  gr.  iv  to  ^j. 

Ter  in  die. 


On  the  fourth  day  the  patient  returns  for  examina- 
tion. With  the  mirror  the  fundus  reflex  is  moderate ; 
the  shadow  moves  slowly  with  the  mirror.  On  trying 
+  2  D.  the  illumination  improves,  and  the  shadows 


RETINOSCOPY  103 

are   more  distinct   and  move  quicker;    +  4  D.  neu- 
tralises the  shadow. 

E.V.  +  3-5D.  =  f. 

L.V.  +  3-5D.=S^. 
o 

This  patient  has^  therefore^  a  total  hypermetropia 
of  +  3*5,  taking  off  +  1  D.  for  the  atropine. 

+  2'5  D.  ordered  for  constant  use. 

Case  3.  Myopia. — A  young  man  eet.  18  complains 
that  he  is  unable  to  see  distant  objects  well. 
K.V.-Aj-3-5D.  =  |. 

L.V.-Ay-8-5D.  =  f- 

After  using  atropine  for  three  days  the  eyes  are 
again  examined.  The  shadows  move  against  the 
movements  of  the  mirror.  —2D.  neutralises  the 
shadow  in  both  eyes. 


R.V.-3  D. 


6 
L.V.-3D.  =  |. 


—  3D.  ordered  for  constant  use. 

Case  4.  Myopia. — Man  a3t.  30  complains  of  seeing 
distant  objects  badly,  but  has  no  difficulty  with  near 
work. 

E.V.g%-2-5D.  =  f. 

L.V.-8V2-5D=|- 

After  three  applications  of  homatropine  and  cocaine 
the  pupils  are  found  to  be  well  dilated.    Retinoscopy  : 
1-5  -1-5 


E. 1-5  L.- 


E.V.-2-25D.  =  f 
L.V.-2-25D.-|, 


1-5 


Order  for  distance  —  2*25  D. 


104  TFE    REFRACTION    OF    THE    EYE 

Case  5.  Compound  Hypermetropic  Astigmatism. — 
A  man  set.  20  : 

R.V.^4-Hm.4D.  =  f. 

Under  atropine,  right  eye  at  distant  types  sees  only 
-f^.  Fundus  reflex  very  dull,  movements  of  shadow 
slow  and  with  the  mirror.  On  putting  up  +  5  D. 
the  reflex  is  much  brighter,  the  edge  of  the  shadow 
distinct,  and  its  movements  quicker.  We  try  +  6, 
7,  8,  9,  and  the  last  lens  gives  a  shadow  against  the 
mirror.  On  trying  the  eye  at  the  distant  type  with 
+  8  D.  |-  and  four  letters  of  f  are  at  once  read. 
No  alteration  in  the  glass  improves  sight. 

Left  eye :  the  fundus  reflex  and  movements  are  the 
same  as  in  the  right.  We  commence  by  trying +  8  D., 
which  we  found  the  other  eye  required.  In  the 
vertical  meridian  the  movement  is  with  the  mirror, 
while  +  9  D.  causes  it  to  move  against.  In  the 
horizontal  meridian  with  +  8  D.  the  shadow  moves 
against  the  mirror,  and  +  7  D.  causes  it  to  move 
with.  We  express  it  thus  : 
+  8D. 

-  +  7D.; 

and  on  trying  the  combination  at  the  distant  type, 

+  7  D.  sp. 

+  1  D.  cy.  axis  horizontal, 
the  patient  is    able    to    read   |-;    on   decreasing  the 
sphere  from  7  D.  to  6'5  D.,  ^  is  read,  so  that  the 
proper  correction  for  this  eye  is — 

-t^6_5  R  sp. 

+  1  D.  cy.  axis  horizontal ; 


EETINOSCOPY  105 

in  this  case^  therefore,  hypermetropia  was  present  in 
one  eye,  compound  hypermetropic  astigmatism  in  the 
other. 

Case  6.  Astigmatism. — Young  woman  set.  17  sees 
with  each  eye  -f^  —  1  B.  =  j\.  Retinoscopy  with- 
out atropine  : 

,-3-5  D.  \-/~^"^* 

ID.  I^./\ 

/    \+lD. 

Ordered  guttle  atropise  sulphatis,  gr.  iv  to  5j,  three 
times  a  day  in  each  eye,  for  three  days ;  then  with 
retinoscopy  the  result  is  : 


2-5  D 
E. 


Em.  =--50^ilP-_  . 

—  2-5  D.  cy.  axis  horizontal,  =  ^. 

_\'/  =      +'50^  =  9. 

^'~      \  -  3D.  cy.  axis  130° 

\  + 1  D. 

After  recovering  from  atropine  the  result  was  con- 
firmed, and  the  following  correction  ordered  to  be 
worn  constantly  : 

E.     -_l_I^-sp^ 

—  2*5  D.  cy.  axis  horizontal.    • 

L.     -3  D.  cy.  axis  130°. 

Case  7.  Mixed  Astigmatism. — Mary  E — ,  £et.  15, 
pupil-teacher,  brought  up  from  Cardiff  about  her 
eyes;  suffers  much  from  headache  and  pain  in  the 
eyeballs,  especially  the  right,  worse  in  the  evenings. 
Has  tried  many  opticians   to    get  spectacles  to  suit 


106  THE    REFRACTION    0¥    THE    EYE 

lier^  but  has  always  been  unable  to  do  so.  R.  V.  3^ 
slightly  improved  with  —  ID.  L.  V.  -3%  also  slightly 
improved  with  —ID.  On  placing  the  patient  in  the 
dark  room,  retinoscopy  at  once  shows  the  case  to  be 
one  of  mixed  astigmatism,  with  the  chief  meridians 
horizontal  and  vertical;  we  proceed  to  correct  each 
meridian,  and  the  result  is  : 


R. 


5D.  [-5D. 

-+1D.        L.— — +  1-5D. 


On  trying  this  combination  before  the  right  eye,  -^-^ 
is  read.     We  express  the  vision  of  right  eye  thus : 
R.-^  +  '50  D.  sp.O  — 6  D.  cy.  axis  horizontal  =  ^%. 

With  the  left  eye  the  combination  gives,  with  the 
cylinder  not  quite  horizontal,  but  slightly  outwards 
and  downwards,  |-. 

L.  3^g  +  1  D.  sp.O  -6  D.  cy.  axis  170° =|^. 

The  patient  remarked  that  she  had  never  seen  things 
so  clearly  before.  The  result  was  very  satisfactory, 
and  was  arrived  at  in  about  ten  minutes,  thus  saving 
an  infinite  amount  of  time  and  trouble,  which  would 
have  been  required  to  work  out  such  a  case  by  any  of 
the  older  methods.  Ordered  guttse  atropioe  sulphatis, 
gr.  iv  to  3J^  three  times  a  day  for  four  days,  when  the 
result  was  : 


|-4D. 
E.— — +2D. 


-4  D. 
— +  2D. 


E.V.-^Q +1-5  D.  sp.O -6  D.  cy.  axis  l75°=f. 
L.V. -g^Q  + 15  D.  sp.C^-e  D.  cy.  axis  170°=-|. 


KETINOSCOPY  107 

In  this  case  tlie  glasses  were  again  tried  after  the 
atropine  was  recovered  from,  and  the  following 
glasses  ordered,  which  were  of  course  to  be  worn 
constantly : 


■^  +  -75  D.  sp.  j^  + 1  D.  sp. 

*  — 6  D.  cy.  axis  175°.  *  — 55  D.  cy.  axis  170' 


>7no 


Case  8.  Astigmatism. — Mr.  C — ,  £et.  24,  has  noticed 
that  for  the  past  few  years  the  eyes  become  very 
tired  at  night,  especially  when  much  writing  or 
reading  has  been  done ;  he  thinks  he  sees  distant 
objects  less  clearly  than  formerly. 

R.  y.  2T  ^^^  improved  with  convex  or  concave 
spheres ;  with  pin-hole  test  -^. 

L.  Y.  -f-g  not  improved  with  convex  or  concave 
spheres;  with  pin-hole  test  y^. 

After  using  atropine  for  four  days,  retinoscopy  gave 
the  following  results : 


1+3  D. 
R.— I— +oD.        L.— 

I 


+  2-5  D. 
— +5  D. 


R.V.  +  2  5  D.  cy.  axis  160'  =  f.         L.V.  +  1-5  D.  axis  cy.  165°- f. 

We  direct  the  patient  to  return  after  the  eifects  of 
the  atropine  have  passed  off,  which  he  does  in  ten 
days;  we  then  try  our  correction,  deducting  +  1  D. 
sphere  for  the  atropine. 

^^      -ID.sp.  6       j^y      -^^D. 6 

+  2-5  D.  cy.  axis  160°  ~  6  •  +  1*5  D.  cy.  axis  165°      5 

This  correction  was  accordingly  ordered  to  be  worn 
constantly. 


108  THE    EEFEACTION    OF    THE    EYE 

Case  9.  Astigmatism. — Sarah  K — ,  set.  21^  com- 
plains that  her  eyes  have  of  late  been  very  painful, 
and  she  has  also  suffered  much  from  headaches, 
which  have  sometimes  ended  with  an  attack  of  sick- 
ness. 


E.V.-,% 

-1^=T^2- 

L.V.-,V2D.=^3-. 

After  atropine,  retinoscopy  gave  : 

E.— 

-1-25  D. 
— +  2D. 

\  :     /    +  2  D. 

/\-2-5D. 

E.V.    +1^!?: 
-2-25  D.c 

y.  axis  horiz. 

^             ■     -4D.cy.axis  125°' 

6 
=  9 

When  the  effects  of  the  atropine  had  passed  off,  the 
correction  which  gave  the  best  results  was  : 

E.V.     -2-25  D.  cy.  axis  horiz.  =^         j^y    +-25D.sp.  =6 

J  b-  —  4D.cy.  axis  125        9 

These  spectacles  were  ordered  to  be  worn  con- 
stantly. 

Case  10.  Simple  Hypermetropic  Astigmatism. — Jane 
Q — ,  set.  11,  has  always  seen  near  objects  badly;  she 
turns  her  head  to  one  side  instead  of  looking  directly 
at  the  object. 

K.  V.  2^  not  improved  with  spheres,  with  pin- 
hole 3%-. 

I^-  ^'  -2T  ^^^  improved  with  spheres,  with  pin- 
hole ^. 

Eetinoscopy  after  atropine  gives  : 

j+2D.  I  +  1-75D. 

E.— — +  6D.  L.— — +  5D. 


RETINOSCOPY  109 

R.V.    +  ^  ^-  ^P-  =_6_      L.V.    +  '"^^  P-  SP-  =_6_ 

+  4  D,  cy.  axis  vert.     1  2  *        '    '    +  3-5  D.  cy.  axis  vert.     ^  ^ ' 

Ordered  for  constant  use  : 
E. V.    +  4  D.  cy .  axis  vert.  =  -j^.     L.V.   +  3-5  D.  cy.  axis  vert.  =  -^. 

Case  11.  Myopic  Astigmatism. — Jane  P — ,  ast.  23, 
always  seen  rather  badly,  and  has  had  a  good  deal 
of  pain  and  discomfort  in  the  eyes  for  the  past  six 
months,  especially  when  using  them  by  gas-light. 
About  a  week  ago  she  noticed,  on  closing  the  left  eye, 
that  the  vision  of  the  right  was  almost  gone,  though 
she  admitted  never  having  tried  them  separately 
before;  occasionally  the  right  eye  turns  outwards. 

E.V.-ij-4D.=^. 
L.V.^-1  D.  =  -,%. 

Homatropine  was  applied  once,  and  at  the  end  of 
half  an  hour  retinoscopy  gave  : 


5-5  D. 
E. 3  D. 


ID. 
-Em. 


With  glasses  : 

E.V. 

L.V. 


-3-5  D.  ^6 

-2-5  D.cy.  axis  175°      12"- 

-•50  D.  sp. ^6^ 

- 1  D.  cy.  axis  5°     6' 


This  correction  was  ordered  for  constant  use. 

Case  12.  Concomitant  Convergent  Strabismus. — 
Mabel  C — ,  set.  9,  commenced  to  squint  with  the  right 
eye  about  the  age  of  four  and  a  half.  Has  never 
worn  glasses. 


E  V  -^-  Ti  V  -^- 


110  THE    REFRACTION    OF    THE    EYE 

On  taking  the  patient  into  the  dark  room  retino- 
scopy  at  once  reveals  the  presence  of  hypermetropic 
astigmatism. 

Grutta3  atropinae  sulpha.tis,  gr.  iv  to  ^j,  was  pre- 
scribed, one  drop  to  be  applied  to  each  eye  three  times 
a  day  for  three  days ;  then  with  retinoscopy : 


E 


,+3 

+  1D. 

J-  .5 

L._—  +4D 
1 

-tS. 

=  _6_ 

axis  vert.     1  2  • 

^■^-  't!^' 

=  6. 
axis  vert.     6(4) 

Prescribed  for  constant  use,  deducting  -|-  *50  D. 
for  atropine : 

+  3  D.  cy.  axis  vert. 
L.    +  3  D.  cy.  axis  vert. 

Case  13.  Right  Constant  Convergent  Concomitant 
Strabismus. — George  A — _,  aet.  5.  Has  squinted  with 
the  right  eye  for  the  past  two  years.  The  angle  of 
the  squint  is  30°.  He  does  not  know  his  letters,  so  is 
ordered  guttae  atropinae  sulphatis  for  four  days.  On 
his  return  the  eyes  appear  almost  straight.  Retino- 
scopy gives : 

+  5                            +4 
E. +  6  L. +  5 

We  deduct  +  1  D.  for  the  over-correction  by  retino- 
scopy, and  +  1  D.  for  the  atropine,  and  order  him  for 
constant  use  : 


RETINOSCOPY  111 

+  3  D.  sp.  +  2  D.  sp. 

^'     +  1  D.  cy.  axis    |  +  1  D.  cy.  axis    | 

The  spectacles  are  made  with  twisted  wire  tem- 
porals to  go  round  the  ear. 

Case  14.  Compound  Myopic  Astigmatism.  —  Miss 
M.  E — _,  sdt.  22,  has  always  been  short-sighted,  but 
thinks  the  sight  has  lately  got  worse. 

Retinoscopy  gives  without  a  mydriatic  : 

-6  -7 


E. 


R. 


L. 


■6 


—  55  D.  sp.  ^6 

—  ID.  cy.  axis  horiz.     9  * 

-6-5  D.  sp.  ^_Q 

—  ID.  cy.  axis  horiz.      1  2  • 


These  glasses  were  ordered  for  constant  use. 
Case  15.  Mixed  Astigmatism. — Miss  L.  U — ,  ast.  54, 
has  worn  glasses  constantly  for  the  past  twenty  years. 


R.V 

2\(2)N.I.S.             L.V.-2\(2)N.I.S. 

inoscopy 

without  a  mydriatic : 

E. 

^-3-5D.                            /-3-5D. 

+  -25  D.  sp.              _ 
^'^'     _4  D.  cy.  axis  170°     g  • 

+  -25D.SP.               _6 
^•^-     -4D.cy.axisl70°     6(4). 

112  THE    REFRACTION    OF    THE    EYE 

Ordered  these  glasses  for  distance ;  the  patient  also 
requires  glasses  for  near  work,  and  reads  best  with 
this  correction  : 

+  2-5  D.  sp. 
^'     -41).  cy.  axis  170°. 
+  2-5  D.  sp. 
•     -4  D.  cy.axis  170°. 

These  were  therefore  prescribed  for  this  purpose. 
Case  16. — G-eorge  M — ,  ast.  23,  complains  of  diffi- 
culty in  reading  and  aching  of  the  eyes. 

E.V.  -j^2-  N.I.S.*  L.V.  -j^g-  N.I.S. 

After  three  applications  of  homatropine  and  co- 
caine, retinoscopy  gives  : 

i-iD.  -ID. 

E.— l—E.  L.— — E. 

I  I 

- 1  D.  cy.  axis  horiz.     6*  •    •  —  1  D.  cy.axis  horiz.     6* 

Ordered  these  glasses  for  constant  use. 
Case  17. — James   C — ,    mt.  48,  has  a  difficulty  in 
reading  at  night. 

E.V.-j^g--l  D.  cy.axis  horiz.  |. 
L.V.-j^g— 1  D.  cy.  axis  horiz.  |. 
Retinoscopy : 


E.— 


E. 


+  1  L.— 


E. 


+  1 


Deducting  +  1  D.  sphere  from  this  result,  which  we 
obtained  with  retinoscopy,  gives  us  : 

—  ID.  cy.  axis  horiz.  for  distance. 
*  N.I.S.  signifies  not  improved  with  spherical  lenses. 


RETINOSCOPY  113 

We  wish  to  add  +  1  D.   sphere  to  this  correction 

for  reading  thus  : 

+ 1  D.  sp.  + 1  D.  sp. 

—  1  D.  cy.  axis  horiz.  '     —ID.  cy.  axis  horiz. 

But  this  is  not  the  simplest  expression  of  the  glass  ; 

it  should  be : 

+  1  D.  cy.  axis  vertical. 

This  patient  therefore  requires  two  pairs  of  glasses : 

—  ID.  cy.  axis  horizontal  for  distance. 
+  1  D.  cy.  axis  vertical  for  reading. 

Case  18.  Myopia  with  Divergent  Strabismus. — Jane 
M — J  set.  22,  has  always  been  short-sighted  since  she 
can  remember,  and  is  now  wearing  —  3' 5  J).,  which 
she  has  worn  constantly  for  the  past  three  years.  The 
left  eye  is  weaker  than  the  right,  and  turns  out  some- 
what, especially  when  she  is  tired. 

E.V..^-6D.  =  f. 

L.V,/„-6D.=-4-. 

Drops  of  homatropine  and  cocaine  were  applied 
three  times  at  intervals  of  ten  minutes.  In  half  an 
hour  the  pupils  being  well  dilated,  the  patient  was 
taken  into  the  dark  room  for  retinoscopy,  with  the 
following  result : 

-5D.                                       -6 
E.— ! 4-5  D.  L. 5  D. 


'  —  •50  D.  cy.  ax.  horiz.      ^' 


j^^-5D.  sp.   ^  ^^ 

—  ID.  cy.  ax.  horiz.     ^  V  3  j  • 


114 


THE    REFRACTION    OF    THE    EYE 


These  glasses  were  prescribed  for  constant  use, 
and  with  these  the  divergence  of  the  left  eye  was 
corrected. 

Case  19.  Mixed  Astigmatism. — James  B — _,  set.  24, 
has  always  seen  badly,  and  is  very  subject  to  head- 
aches, which  affect  the  occipital  region  chiefly.  These 
headaches  are  always  made  worse  by  reading,  and 
frequently  come  on  after  a  long  spell  of  near  work. 
Has  worn  glasses  constantly  for  the  past  six  years, 
but  the  present  ones  are  not  comfortable. 


E.V.6%N.LS. 


L.V.-e^N.I.S. 


Retinoscopy  without  a  mydriatic  : 


+  1D. 


-3D. 


+  1D. 
-3D. 


-P  y  +  1  D.  sp.  ^   6_ 

■    *-4D.  cy.  ax.  165°     12- 

L.V.  "^  ^  ^-  ^P- 

— 4  D.  cy.  ax.  horiz. 


6 

1  2 


Homatropine  and  cocaine  to  both  eyes ;  after  three 
applications  the  pupils  are  found  fully  dilated. 
Retinoscopy : 


E. 


+  3 


.2D. 


^  +  3  D. 
1-5D. 


y  +2D.  sp.  ^6^    ^ 

-5D.cy.  ax.l65°      6(3). 
j^y  -t- 2  D.  sp.  ^6^ 

*  — 4-5  D.  cy.  ax.  horiz.     ^' 


prescribed  for  constant  use. 


RETINOSCOPY  1 1 5 

T,   +  15  D.  sp. 

'  —  5  D.  cy.  ax.  165''. 
T    +15  D.  sp. 

—  4*5  D.  cy.  ax.horiz. 

Case  20. — Mrs  H. — ,  aet.  50,  has  worn  glasses  for 
near  work  for  the  past  six  years.    During  the  last  few 
months  the  distant  vision  has  deteriorated,  and  the 
present  reading  glasses  are  not  satisfactory. 
R.V.-,%Hm.lD.  =  f  ,g 


L.V.-j^g-Hm.  ID.  =  f  )  ^ 
Beads  best  with  +  3  D.     Ordered  +  1   D.  for  dis- 
tance, and  +  3  D.  for  near  work. 

Case   21.    Astigmatism  with  Presbyopia. — Mr.  N — , 
aet.  48,  sees  badly  at  all  distances : 

B,.Y.-^^+15  D.  cy.  ax.  vert.  =  f. 
L.V.-j^g- +  1-5  D.  cy.  ax.  vert.  =  f . 

Reads  best  with  : 

+ 1  D.  sp. 

+  15  D.  cy.  ax.  vert. 
Ordered,  therefore,  two  pairs  of  glasses,  one  for  dis- 
tance, the  other  pair  for  near  work. 

Case   22.  Myopic   Astigmatism  with   Presbyopia. — 
Miss  K — ,  set.  60,  has  difficulty  in  doing  near  work. 
R.V.-j^g— 1  D.  cy.  ax.  lioriz.  =  |^. 
L.V.^g--!  D.  cy.  ax.  horiz.  =  |-. 
Retinoscopy  without  a  mydriatic  : 

R. E.  L.—  — E. 

Ordered   two   pairs   of   glasses.     Distance  —  ID. 
cy.   ax.  horizontal. 


116 


THE    EEFRAOTION    OP    THE    EYE 


Reading  and  near  work  : 

+  2  D.  sp^ 

+  1  D.  cy,  ax.  vertical. 

In  most  cases  thus  worked  out  tlie  glasses  may  be 
ordered  at  once,  without  waiting  for  the  effects  of 
the  atropine  to  pass  off — in  fact,  experience  teaches 
that  in  children,  it  is  a  good  plan  to  continue  the 
atropine  until  the  spectacles  have  been  made ;  re- 
membering when  ordering  the  correction  that  in 
hypermetropia  and  hypermetropic  astigmatism  the 
spherical  glass  will  require  slightly  diminishing,  usually 
about  "50  D. ;  in  myopia  and  myopic  astigmatism  the 
spherical  glass  has  to  be  slightly  increased. 


HYPEEMETROPIA 


117 


CHAPTER  VI 


HYPEEMETROPIA 


Hypermetropia  (H.)  {'Yirlp,  in  excess;  juiTpov,  mea- 
sure ;  and  wxp,  eye)  may  be  defined  as  a  condition  in 
which  the  antero-posterior  axis  of  the  eyeball  is  so 
shorty  or  the  refracting  power  so  low,  that  parallel 
rays  are  brought  to  a  focus  behind  the  retina  (the 
accommodation  being  at  rest).  In  other  words,  the 
focal  length  of  the  refracting  media  is  greater  than 
the  length  of  the  eyeball. 

Fig.  66. 


Parallel  rays  focus  at  h  behind  the  retina ;  those  coming  from 
the  retina  emerge  as  diverging  rays,  d,  e. 


In  the  passive  hypermetropic  eye,  therefore,  paral- 
lel rays  c  and  g  come  to  a  focus  behind  the  eye  at  6, 
forming  on  the  retina  at  a  a  circle  of  diffusion  instead 
of  a  point.     Rays  coming  from  the  retina  of  such  an 


118  THE    REFRACTION    OF    THE    EYE 

eye  emerge  having  a  divergent  direction  (d  and  e)  ; 
these  rays^  if  prolonged  backwards,  will  meet  at  k, 
which  is  the  punctum  remotum,  and  this  point  being 
situated  behind  the  eye  is  called  negative. 

The  distance  of  the  punctum  remotum  behind  the 
eye  will  equal  the  focus  of  the  convex  lens  which 
corrects  the  hypermetropia ;  thus,  supposing  the  p.  r. 
situated  20  cm.  behind  the  retina  (^^'^  =  5),  5  D.  will 
be  the  convex  glass  which  will  render  parallel  rays  so 
convergent  that  they  will  focus  on  the  retina,  or 
cause  rays  from  the  retina  to  be  parallel  after  passing 
through  it ;  to  be  mathematically  correct,  allowance 

Fig.  67. 


Parallel  rays  fociissed  on  the  retina  by  accommodation.  The 
dotted  line  shows  the  lens  more  convex  as  a  result  of  the 
contraction  of  the  ciliary  muscle. 

must  be  made  for  the  distance  between  the  cornea 
and  the  convex  lens ;  thus,  for  instance,  if  the  lens 
be  placed  20  mm.  from  the  cornea,  then  the  exact 
amount  of  hypermetropia  which  the  -f  5  D.  glass  will 
correct  will  be  : 

1000       ^  1000  ^  K... 
200  -  20        180 
In  low  degrees  of  hypermetropia  the  difference  is 


HYPERMETROPIA  119 

SO  slight  as  to  be  unimportant ;  in  the  higher  degrees 
the  difference  is  considerable. 

The  hypermetropic  eye  at  rest  is  only  able  to 
bring  convergent  rays  to  a  focus  on  the  retina.  All 
rays  in  nature  are  divergent,  some  so  slightly  so,  that 
when  coming  from  a  distant  object  they  are  assumed 
to  be  parallel.  Rays  can  be  made  convergent  by 
passing  them  through  a  convex  lens  placed  in  front 
of  the  eye  ;  or  the  refraction  of  the  dioptric  system 
may  be  increased  by  the  accommodation,  so  that 
parallel  rays  may  then  focus  on  the  retina  of  a  hyper- 
metropic eye. 

Therefore  a  hypermetrope  with  relaxed  accommo- 
dation sees  all  objects  indistinctly.  The  hypermetropic 
eye  has  to  use  some  of  its  accommodation  for  distance, 
so  starts  with  a  deficit  for  all  other  requirements, 
equal  to  the  amount  of  hypermetropia. 

Fig.  68. 


Parallel  rays  rendered  so  convergent  by  passing  through  a  convex 
lens  that  they  focus  on  the  retina. 

Thus,  supposing  an  individual  hypermetropic  to 
the  extent  of  four  dioptres,  and  possessing  6  D.  of 
accommodation,  he  will,  by  the  exercise  of  this  power 


120  THE    EEFEACTTON    OF    THE    EYE 

to  the  extent  of  4  D.^  be  able  to  bring  parallel  rays 
to  a  focus  on  the  retina^  and  so  see  distant  objects 
clearly;  this  leaves  him  2  D.  of  accommodation  for 
near  objects^  which  will  bring  his  near  point  to  50 
cm.,  a  distance  at  which  he  will  be  unable  to  read 
comfortably. 

Besides,  it  must  be  remembered  that  only  a  part  of 
the  accommodation  can  be  used  for  sustained  vision, 
fatigue  soon  resulting  when  the  whole  of  the  accom- 
modation has  to  be  put  in  force. 

^J'he  following  diagram  is  intended  to  show  the 
amount  of  accommodation  possessed  by  a  hyperme- 
trope  of  3D,;  each  space  represents  a  dioptre,  and 
the  thick  white  lines  drawn  through  the  spaces  give 

Fig.  69. 
Dioptres. 


HHHB 


the  amplitude  of  accommodation  for  different  ages  as 
given  on  the  left  of  the  diagram.  The  figures  above 
indicate  the  number  of  dioptres,  and  those  below,  the 
near  point  for  each  increasing  dioptre  of  accommoda- 
tion. 


HYPERMETROPIA  121 

The  amount  of  hypermetropia  is  calculated  and 
expressed  by  that  convex  glass  whicli  makes  parallel 
rays  so  convergent  that  they  meet  on  the  rods  and 
cones  of  the  retina,  the  accommodation  being  sus- 
pended. 

The  commonest  amount  of  error  is  about  2  D. 
Small  degrees  may  require  some  trouble  to  discover, 
and  sometimes  can  only  be  found  out  after  the  eye  has 
been  atropized. 

Hypermetropia  is  divided  into  latent  and  manifest. 
The  manifest,  Bonders  subdivides  into  absolute,  rela- 
tive, and  facultative  : 

Absolute,  when  by  the  strongest  convergence  of 
the  visual  lines  accommodation  for  parallel  rays  is 
not  attained — in  other  words,  when  distant  vision  is 
impaired;  this  variety  is  seldom  met  with  in  young 
people. 

Relative,  when  it  is  possible  to  accommodate  for  a 
near  point,  by  converging  to  a  point  still  nearer, — in 
fact,  by  squinting. 

Facultative,  when  objects  can  be  clearly  seen  with 
or  without  convex  glasses. 

In  young  people  the  hypermetropia  may  be  facul- 
tative, or  relative,  becoming  in  later  life  absolute. 

Causes  of  Hypermetropia  : 

].  The  antero-posterior  diameter  of  the  eyeball 
is  too  short  (axial  hypermetropia).  This  is 
by  far  the  most  common  cause,  and  is  con- 
genital. 


122  THE    REFRACTION    OF    THE    EYE 

2.  A  flattened  condition  of  the  cornea,  the  result 

of  disease  or  occurring  congenitally. 

3.  Absence  of  the  lens  (aphakia). 

4.  Detachment  or  protrusion  of  the  retina,  owing 

to  a  tumour  or  exudation  behind  it. 

5.  A  diminution  in  the  index  of  refraction  of  the 

aqueous,  lens,  or  vitreous. 


Hypermetropia,  therefore,  is  usually  due  to  shorten- 
ing of  the  axis  of  the  eyeball. 

The  following  table  shows  the  amount  of  shortening 
for  each  dioptre  of  hypermetropia,  the  axial  line  in 
emmetropia  being  estimated  at  22*824  mm. 


or  -5  of  D. 

of  H.  there  is  a  diminution  in  the  axial  line  of  -16  mm 

ID 

)»                            }>                             } 

•31    „ 

1-5 

,,                            „  .                            , 

•47    „ 

2- 

„                             „                              , 

•62    „ 

2-5 

j>                            >f                             } 

•77    „ 

3- 

,,                            }>                             J 

•92    „ 

3-5 

„ 

1-06    „ 

4- 

,,                             „                              J 

1-22    „ 

4-5 

,,                             „                              J 

1^4      „ 

5- 

>»                            »                             i 

1-6      „ 

6- 

„ 

1^9      „ 

7- 

„                             „                              , 

22      „ 

8- 

„                             , 

2-6      „ 

9- 

„                             „                              J 

29      „ 

10- 
TTvnprT 

riAfvn-niji,    i«    \\\t  -far  fliA    mn<2f,    fvp 

3-2      „ 

tion  of  the  refraction.     It  may  be  looked  upon  as  a 

congenital  defect;    frequently  also  it   is  hereditary, 

several  members  of  the  same  family  suffering  from  it. 

Hypermetropia  is  usually  due  to  an  arrest  of  deve- 


HYPERMETROPIA  123 

lopment,  which  varies  from  the  slightest  degree  to 
the  extreme  condition  known  as  "  microphthalmos." 

The  following  are  some  of  the  chief  points  in  which 
the  hypermetropic  differs  from  the  emmetropic  eye : — 
the  eye  looks  small,  being  less  than  the  normal  in  all 
its  dimensions,  especially  the  antero-posterior ;  the 
sclerotic  is  flat,  and  makes  a  strong  curve  backwards 
in  the  region  of  the  equator,  which  can  easily  be  seen 
on  extreme  convergence,  or  can  be  felt  by  the  finger. 
The  lens  and  iris  are  more  forward,  the  anterior 
chamber  is  shallow,  and  the  pupil  small ;  the  centre  of 
rotation  of  the  eye  is  relatively  further  back,  while 
the  angle  a,  which  is  formed  between  the  visual  and 
optic  axis,  is  invariably  greater,  averaging  about  7° 
(see  p.  202).  The  result  of  the  large  angle  a  in 
hypermetropia  is  that  the  eyes  often  have  an  appear- 
ance of  divergence,  which  has  sometimes  been  mis- 
taken for  real  divergence;  whereas  in  myopia  the 
small  angle  gives  to  the  eyes  an  appearance  of  con- 
vergence. 

The  ciliary  muscle,  upon  the  action  of  which  the 
accommodation  depends,  is  much  larger  than  in  em- 
metropia,  the  anterior  portion,  which  consists  chiefly 
of  circular  fibres,  being  especially  developed,  no 
doubt  hypertrophied  by  the  constant  state  of  con- 
traction in  which  it  is  kept.  This  contraction  is 
called  into  action  by  the  instinctive  desire  for  clear 
images  which  all  eyes  possess,  the  accommodation 
having  to  be  used  for  distant  as  well  as  for  near 
objects.  Another  result  of  the  constant  and  exces- 
sive accommodation,  is  that  its  linked  function — the 


124  THE    REPEACTION    OF    THE    EYE 

convergence — is  liable  also  to  be  used  in  excess ;  in 
this  case  an  object  at  a  certain  distance  being  accom- 
modated for^  one  eye  will  be  directed  to  the  object, 
while  the  other,  taking  up  the  excessive  convergence, 
will  be  directed  inwards,  and  so  a  convergent  stra- 
bismus will  be  produced.  To  fully  understand  how 
this  convergent  strabismus  becomes  developed,  I 
must  refer  the  reader  to  the  chapter  on  that  subject 
(Chap.  X). 

When  the  hypermetropia  is  of  high  degree  the 
optic  nerve  is  smaller,  and  contains  fewer  fibres,  so 
that  the  visual  acuteness  in  these  cases  may  be  below 
the  normal. 

Sometimes  the  face  also  has  a  characteristic  appear- 
ance, being  flat-looking,  with  depressed  nose,  the  orbits 
being  shallow,  and  the  eyes  set  far  apart.  Frequently, 
however,  there  is  no  distinctive  physiognomy. 

The  hypermetropic  eye  is  very  liable  to  asymmetry, 
as  will  be  shown  when  speaking  of  astigmatism. 

Symptoms  of  Hypermetropia. — The  patient  usually 
sees  well  at  a  distance,  but  has  difficulty  in  main- 
taining clear  vision  for  near  objects ;  and  since  the 
hypermetropia  can  be  more  or  less  corrected  by  ac- 
commodation, if  the  error  be  of  a  low  degree  (as  2  or 
3  D.),  no  ill  effects  may  for  some  time  be  noticed;  at 
length,  however,  a  point  is  reached  when  the  accom- 
modation is  not  equal  to  long-sustained  efforts  of 
reading  and  near  work,  then  accommodative  asthe- 
nopia is  the  result  (p.  226).  This  is  especially  liable 
to  show  itself  after  an  illness,  or  if  the  patient's 
health  has  deteriorated  from  over-work,  anxiety,   or 


HYPERMETKOPIA  125 

other  causes.  He  then  complains  that  after  working 
or  reading  for  some  time,  especially  during  the 
evenings,  the  type  becomes  indistinct,  and  the  letters 
run  together ;  after  resting  awhile  the  work  can  be 
resumed,  to  be  again  shortly  laid  aside  from  a 
repetition  of  the  dimness :  the  eyes  ache,  feel  weak, 
water,  etc.,  frequently  headache  supervenes  ;  there  is 
a  feeling  of  weight  about  the  eyelids,  and  a  difficulty 
in  opening  them  in  the  morning.  When  the  hyper- 
metropia  is  of  high  degree,  the  patient  may  be  said 
by  his  friends  to  be  short-sighted,  because  when 
reading  he  holds  the  book  close  to  his  eyes ;  by 
doing  this  he  increases  the  size  of  his  visual  angle, 
and  thus  gets  larger  retinal  images ;  this  is  counter- 
balanced by  an  increase  in  the  circles  of  diffusion,  but 
as  the  pupils  also  contract  by  approaching  the  book  to 
his  eyes,  some  of  these  are  cut  off;  so  that  the  advan- 
tage is  in  favour  of  holding  the  book  close,  especially 
as  the  patient  is  probably  not  accustomed  to  clear, 
well-defined  images.  In  some  cases  the  ciliary 
muscle  contracts  in  excess  of  the  hypermetropia,  so 
that  parallel  rays  focus  in  front  of  the  retina,  and  the 
patient  therefore  presents  many  of  the  symptoms  of 
myopia :  we  should  always  be  on  our  guard  against 
such  cases.  The  manner  in  which  the  patient  reads 
the  distant  type  is  often  a  guide  to  us  in  hyperme- 
tropia; he  takes  a  considerable  time  to  make  out  each 
line,  and  yet,  if  not  hurried,  eventually  reads  the 
whole  correctly.  On  looking  at  the  eyes  one  notices 
that  they  are  red  and  weak,  the  lids  look  irritable, 
and  on  eversion  the  conjunctiva  is  hyperaemic,  espe- 


126 


THE    REFRACTION    OF    THE    EYE 


cially  that  of  the  lower  lids,  while  the  papillae  are 
frequently  enlarged ;  the  edges  of  the  lids  sometimes 
become  inflamed  and  thickened.  All  these  symptoms 
are  probably  the  commencement  of  troubles  which,  if 
allowed  to  go  on,  may  develop  into  conjunctivitis, 
blepharitis,  derangements  of  the  lacrymal  apparatus, 
etc., — this  much  we  can  see ;  how  much  more  injurious 
must  be  the  changes  which  are  liable  to  take  place  in 
the  interior  of  the  eyeball  from  prolonged  hyperasmia  ! 
It  cannot  be  too  forcibly  insisted  on,  that  in  all  oph- 
thalmic cases,  except  those  of  an  acute  character,  the 
refraction  should  be  taken  and  recorded  as  a  matter 
of  routine,  since  complaints  which  prove  very  intract- 
able are  often  easily  and  quickly  cured  when  the 
proper  glasses  have  been  prescribed. 

As  the  patient  advances  in  age  he  will  become  pre- 
maturely presbyopic,  so  that  at  thirty-five  he  may 
sufPer  from  the  same  discomforts  as  an  emmetrope 
of  fifty. 

To  test  the  hypermetropia  and  measure  the  amount; 
we  commence  by  taking  the  patient^s  visual  acute- 
ness,  each  eye  separately ;  having  found  that  they 
are  alike  in  their  refraction,  we  try  the  two  together ; 
stronger  glasses  being  often  borne  when  both  eyes  are 
used,  than  when  one  is  excluded  from  vision. 

The  strongest  convex  glass  with  which  he  is  able  to 
read  f ,  or  with  which  he  gets  the  greatest  acuteness 
of  vision,  is  the  measure  of  the  manifest  hyperme- 
tropia (Hm.).  This  is  not,  however,  the  total  hyper- 
metropia, for  if  the  accommodation  be  paralysed  by 
applying  a  solution  of  atropige  sulphatis,  gr.  iv  to  3j, 


HYPEEMETROPFA  127 

three  times  a  day  for  four  days  (when  we  may  feel 
sure  that  not  the  least  vestige  of  accommodation 
remains),  a  much  stronger  glass  can  be  tolerated,  and 
will  be  required  to  enable  the  patient  to  read  ^.  This 
strong  glass  represents  the  total  hypermetropia,  the 
additional  amount  to  that  found  as  Hm.  being  called 
late7it   (HI.). 

The  following  plan  is  an  excellent  one  for  measur- 
ing the  manifest  hypermetropia.  Place  in  spectacle- 
frames  before  the  eyes  such  convex  lenses  as  over- 
correct  the  Hm.  ( +  4  D.  will  usually  do  this)  ;  then 
hold  in  front  of  these,  weak  concave  glasses,  until  we 
find  the  weakest,  which  thus  held  in  front  of  +  4  D. 
enables  f  to  be  read;  the  difference  between  the 
glasses  is  then  the  measure  of  the  Hm.  By  this  plan 
the  ciliary  muscle  is  encouraged  to  relax,  and  we  get 
out  a  larger  amount  of  manifest  hypermetropia  than 
is  obtained  by  the  ordinary  method.  Thus,  supposing 
—  2D.  the  weakest  glass  which,  held  in  front  of  the 
convex  4  D.,  enables  the  patient  to  read  f,  -+  2D. 
is  the  measure  of  the  Hm.  ( +  4  D.)  +  (—  2  D.)  = 
+  2D. 

As  age  advances  the  accommodation  diminishes,  and 
the  latent  hypermetropia  becomes  gradually  manifest. 
Thus  a  person  may  have  6  D.  of  hypermetropia  latent 
at  ten  years  of  age,  three  of  which  may  have  become 
manifest  at  thirty-five,  and  the  whole  of  it  at  about 
sixty-five  or  seventy,  when  the  total  hypermetropia  is 
represented  by  the  manifest. 

With  the  advance  of  age  certain  changes  take 
place   in   the    structure    of   the    crystalline   lens,   by 


128  THE    REFRACTION    OF    THE    EYE 

which  its  refraction  becomes  diminished.  This  change 
takes  place  in  all  eyes,  and  at  a  regular  rate  ;  thus  at 
fifty-five  the  refraction  has  diminished  '25  D._,  at 
sixty-five  '75  J).,  at  seventy  1  D._,  and  at  eighty  as 
much  as  2*5  D.  Hypermetropia  when  thus  occurring 
in  eyes  previously  emmetropic  is  styled  acquired  hyper- 
metropia, in  contradistinction  to  the  congenital  form, 
which  is  called  original  hypermetropia. 

The  normal  refraction  of  the  eye  in  early  child- 
hood is  hypermetropic ;  some  remain  so,  a  con- 
siderable number  become  emmetropic  as  they  get 
older,  and  a  certain  percentage  of  these  pass  on 
to  myopia. 

In  the  diagnosis  and  estimation  of  hypermetropia 
several  methods  are  useful.  We  first  estimate  the 
acuteness  of  vision,  remembering  that  being  able  to 
read  |^  does  not  exclude  hypermetropia,  and  that  we 
must  in  all  cases  try  convex  glasses ;  and  if  the  same 
letters  can  be  seen  with  as  without  them,  then  the 
patient  certainly  has  hypermetropia,  and  the  strongest 
convex  glass  with  which  he  sees  them  is  the  measure 
of  his  Hm. 

We  next  proceed  to  retinoscopy  ;  with  the  plane 
mirror  we  get  a  shadow  moving  with  the  mirror  :  the 
quicker  the  movement  and  the  brighter  its  edge,  the 
lower  is  the  degree  of  hypermetropia  (see  p.  87). 

With  the  ophthalmoscope  by  the  indirect  method  of 
examination,  the  image  of  the  disc  is  larger  than 
in  emmetropia,  and  diminishes  on  withdrawing  the 
objective  from  the  eye   (p.  69). 

With  the  mirror  alone  at  a  distance,  an  erect  image 


HYPERMETROPIA  129 

of  the  disc  is  seen,  which  moves  in  the  same  direction 
as  the  observer's  head  (p.  72). 

By  the  direct  method  the  accommodation  of  the 
observer  and  observed  being  relaxed,  a  convex  glass 
is  necessary  behind  the  ophthalmoscope,  to  enable  the 
observer  to  bring  the  diverging  rays  from  the  observed 
eye  to  a  focus  on  his  retina;  the  strongest  convex  glass 
with  which  it  is  possible  to  see  the  details  of  the 
fundus  clearly,  is  the  measure  of  the  total  hyperme- 
tropia   (Fig.  47). 

The  treatment  of  hypermetropia  consists  obviously 
in  prescribing  such  convex  glasses  as  will  give  to  rays 
passing  through  them  an  amount  of  convergence,  so 
that  they  will  meet  on  the  retina  without  undue  accom- 
modation. It  might  be  thought  that,  having  obtained 
the  measure  of  the  total  hypermetropia,  nothing  re- 
mained but  to  give  such  positive  glasses  as  exactly 
neutralise  the  defect,  and  that  we  should  then  have 
placed  the  eye  in  the  condition  of  an  emmetropic  one. 
Such  at  first  was  thought  to  be  the  case,  though  it 
is  by  no  means  so,  because  persons  who  have  been 
accustomed  to  use  their  accommodation  so  constantly, 
both  for  near  and  distant  objects,  as  hypermetropes 
have  to,  possess  very  large  ciliary  muscles  which 
they  cannot  suddenly  completely  relax ;  possibly  also 
the  elasticity  of  the  lens  capsule  is  somewhat  im- 
paired. 

In  children  and  patients  under  twenty  years  of  age 
it  is  much  better  to  atropize  them  at  the  first,  and  so 
measure  once  and  for  all  the  amount  of  total  hyper- 
metropia ;  otherwise  it  will  frequently  be  found  that 


130  THE    REFRACTION    OF    THE    EYE 

the  spectacles  have  to  be  constantly  changed^  the 
asthenopia  is  unrelieved,  and  probably  the  patient  has 
to  be  atropized  after  all,  or  becomes  dissatisfied  and 
goes  off  to  some  one  else.  Another  reason  in  favour 
of  atropine  is,  that  with  it  we  cannot  possibly  mistake 
cases  of  spasm  of  the  ciliary  muscle  in  hypermetropia 
for  myopia,  which  might  otherwise  happen,  since  the 
spasm  causes  the  lens  to  become  so  convex  that 
parallel  rays  are  even  made  to  focus  in  front  of  the 
retina,  thus  simulating  myopia. 

It  must  always  be  borne  in  mind  that  it  is  dangerous 
to  atropize  patients  above  the  age  of  thirty-five,  many 
well-marked  cases  of  "  glaucoma  "  having  been  traced 
to  the  use  of  this  drug ;  moreover  as  age  advances  the 
latent  hypermetropia  gradually  becomes  manifest,  so 
that  the  necessity  for  paralysing  the  accommodation 
becomes  less. 

There  exists  some  difference  of  opinion  among 
ophthalmic  surgeons  as  to  the  amount  of  the  total 
hypermetropia  we  ought  to  correct ;  some  give  such 
glasses  as  neutralise  the  manifest  hypermetropia 
only,  while  others,  after  estimating  the  total,  deduct 
perhaps  1  D.  from  this.  It  will  be  found  that  patients 
vary  much  as  to  the  amount  of  correction  which  is 
most  comfortable  for  them. 

A  good  practical  rule  is  to  prescribe  such  glasses 
for  reading  as  correct  the  manifest  and  one  third  of 
the  latent  hypermetropia. 

For  example,  a  child  having  5  D.  of  hyperme- 
tropia of  which  2  only  are  manifest,  will  require 
+  3  D.  for  reading.     At   the  age  of  twenty,  about 


HYPEEMETROPIA  131 

3  D.  will  have  become  manifest^  and  tlie  patient  will 
then  want  +  3-75  D. ;  at  forty,  4  D.  will  be  manifest, 
and  he  may  then  be  able  to  use  his  full  correction. 

Hence  it  will  be  seen  that,  as  age  advances,  the 
spectacles  will  have  occasionally  to  be  changed  for 
stronger  ones,  as  the  latent  hypermetropia  gradually 
becomes  manifest. 

The  question  arises,  should  spectacles  be  worn  con- 
stantly or  only  for  near  work  ?  So  long  as  distant 
objects  (|-)  can  be  seen  comfortably  without  them, 
their  use  is  unnecessary  except  for  reading  and  near 
work;  this  is  generally  the  case  in  young  persons 
where  the  hypermetropia  does  not  exceed  3  or  4  D. 
When  a  convex  glass  improves  distant  vision,  then 
such  can  be  constantly  worn ;  somewhat  stronger 
glasses  will  be  required  for  reading  after  the  age  of 
forty-five. 

The  disadvantage  of  using  spectacles  constantly  is, 
that  after  wearing  them  for  some  time  the  patient 
finds  he  is  unable  to  see  without  them,  which  is  a 
serious  inconvenience ;  so  that  the  plan  is  not  to  give 
spectacles  for  constant  use  until  the  hypermetropia 
has  become  relative  or  absolute. 

In  cases  of  concomitant  squint,  spectacles  which 
correct  the  hypermetropia  are  to  be  worn  constantly, 
and  here  our  object  must  be  to  give  as  near  the  full 
correction  as  is  consistent  with  the  patient\s  comfort ; 
this  we  can  only  find  out  by  experiment  in  each  case. 
The  best  plan  is  to  measure  under  atropine  the  total 
hypermetropia,  deduct  1  D.,  and  give  this  correction 
for  constant  use  :  the  reason  for  making  this  deduction 


132  THE    REFEACTION    OF    THE    EYE 

is  that  the  ciliary  muscle  is  never  so  completely  relaxed 
as  when  under  atropine. 

Convergent  strabismus  and  asthenopia,  two  of  the 
most  frequent  results  of  hypermetropia,  will  be  treated 
of  in  Chapters  X  and  XI. 

See  Cases  1  and  2,  p.  102  ;  also  23,  25,  and  30, 
p.  244. 

Aphakia 

Aphakia  {'  A,  priv ;  (JyaKOQ,  lens)  is  the  name  given 
to  that  condition  of  the  eye  in  which  the  lens  is  absent. - 
There  are  several  causes,  by  far  the  most  frequent 
being  some  form  of  cataract  operation.  Besides  this 
aphakia  may  be  caused  by  dislocation  of  the  lens 
from  injury,  or  dislocation  may  occur  spontaneously, 
and  this  is  probably  the  cause  of  those  congenital 
cases  where  no  lens  can  be  seen. 

Aphakia  necessarily  converts  the  eye  into  a  very 
hypermetropic  one.  The  length  of  the  eyeball  which 
would  be  required  (the  curvature  of  the  cornea  being 
normal  and  the  lens  absent)  to  bring  parallel  rays  to 
a  focus  on  the  retina  is  30  mm.,  whereas  normally  the 
antero-posterior  diameter  of  the  eyeballs  is  only  about 
22-8  mm. 

To  test  aphakia :  when  a  bright  flame  is  held  in 
front  of  and  a  little  to  one  side  of  a  normal  eye,  three 
images  of  the  flame  are  formed,  one  erect  on  the 
cornea,  another  erect  on  the  anterior  surface  of  the 
lens,  and  a  third  inverted,  formed  on  the  posterior 
surface  of  the  lens.  On  moving  the  flame  up  and 
down,  the   erect   images   move  with  it,  and   the  in- 


APHAKIA  -  133 

verted  one  in  tlie  opposite  direction.  In  aphakia 
two  of  tliese  images  are  absent^  viz.,  those  formed  on 
the  two  surfaces  of  the  lens. 

Treatment. — Strong  convex  glasses  will  be  required 
to  take  the  place  of  the  absent  lens,  the  previous  re- 
fraction of  the  eye  of  course  influencing  their  strength. 
In  hypermetropia,  stronger  glasses  will  be  required; 
in  myopia,  weaker. 

The  convex  glass  usually  required  by  an  eye  pre- 
viously emmetropic,  to  bring  parallel  rays  to  a  focus 
on  the  retina  is  from  10  to  13  D. 

As  every  trace  of  accommodation  is  lost  with  the 
lens,  stronger  glasses  will  be  required  for  reading  or 
near  work,  and  to  find  out  the  necessary  glass  for  a 
certain  distance,  we  have  only  to  add  to  the  distance 
glass  one  whose  focal  length  equals  the  distance  at 
which  we  wish  our  patient  to  see.  Thus,  if  he  require 
+  10  D.  for  distance,  and  wish  to  see  to  read  at  25 
cm.,  we  add  +  4  D.  to  his  other  glass,  and  the  result- 
ing +  14  D.  will  adapt  the  e3'e  for  25  cm. 

The  patient  may  be  taught  a  sort  of  artificial 
accommodation  by  moving  the  spectacles  along  the 
nose,  nearer  or  further  from  the  eyes,  his  working 
point  being  thereby  moved  away  or  brought  nearer 
to  him. 

In  correcting  aphakia  it  will  often  be  found  that 
the  vision  is  below  the  normal.  Frequently  also  there 
is  some  astigmatism,  especially  in  cases  after  cataract 
extraction. 

See  Case  36,  p.  257. 


134  THE    REFRACTION    OF    THE    EYE 


CHAPTER  VII 

Myopia  (m.) 

Myopia  (Muw,  I  close :  wi//,  the  eye),  or  sliort-siglit, 
is  the  opposite  condition  to  hypermetropia. 

We  saw  that  the  hypermetropic  eyeball  was  too 
short,  so  that  parallel  rays  focussed  behind  the  retina; 
it  is  therefore  not  adapted  to  any  real  distance,  be- 
cause in  order  to  see  any  object  clearly,  it  is  necessary 
that  the  defect  should  be  corrected  either  by  the 
accommodation  or  by  means  of  a  convex  glass.  Now 
in  myopia,  although  the  eyeball  is  too  long  to  allow 
of  distant  objects  being  seen  clearly,  it  is  perfectly 
adapted  for  near  vision,  so  that  a  low  degree  of 
myopia  may  not  be  a  very  serious  disadvantage. 

We  spoke  of  hypermetropia  as  congenital,  due  to 
an  arrest  of  development;  myopia  is  an  acquired 
defect,  and  may  be  looked  upon  as  an  effort  of  nature 
to  adapt  the  eye  to  near  objects,  as  a  result  of  civili- 
sation and  its  incessant  demands  on  near  vision. 

Myopia  is  peculiar  to  the  human  race,  and  is  met 
with  much  more  frequently  in  civilised  than  in  un- 
civilised races. 

Low  degrees,  such  as  1  D.,  may  have  no  very  serious 


MYOPIA  135 

drawbacks,  because  although  the  full  visual  acuteness 
can  only  be  obtained  by  the  help  of  concave  glasses, 
many  people  go  half  through  life,  playing  cricket, 
tennis,  shooting,  etc.,  without  finding  out  the  defect ; 
their  near  vision  is  really  better  than  that  of  the 
emmetrope,  for  they  obtain  larger  retinal  images,  and 
they  have  to  accommodate  less ;  against  these  advan- 
tages it  may  be  stated  that  many  myopes  suffer  from 
asthenopia,  the  result  of  disturbance  of  the  harmony 
between  the  two  functions,  accommodation  and  con- 
vergence, though  this  disturbance  will,  of  course,  be 
more  marked  in  the  higher  degrees  of  ametropia. 

Medium  degrees  of  myopia,  from  2  to  6  D.,  are  ex- 
ceedingly common ;  the  visual  defects  are  more  pro- 
nounced, and  it  becomes  necessary  to  use  glasses  for 
many  things :  often  they  have  to  be  worn  constantly. 
Such  patients  are  liable  to  suffer  from  asthenopia,  or 
from  divergent  strabismus  and  its  accompanying  evil 
— loss  of  binocular  vision. 

The  higher  degrees  of  myopia  which  increase 
steadily  and  constantly  from  an  early  stage,  reaching 
often  a  very  high  degree,  and  carrying  in  its  wake 
damage  and  destruction  to  important  ocular  tissues, 
must  be  looked  upon  as  a  serious  disease ;  it  is  desig- 
nated by  the  name  progressive  myopia. 

We  must  now  refer  to  the  optical  condition  of  the 
myopic  eye. 

Parallel  rays,  falling  on  a  myopic  eye,  focus  in 
front  of  the  retina,  cross  and  form  a  circle  of  diffusion 
(Fig.  70),  in  place  of  a  clear  image. 

Only  divergent  rays  focus  on  the  retina,  and  hence 


136 


THE    EEFRACTION    OF    THE    EYE 


it  is  necessary  that  the  object  looked  at  be  brought 
so  near^  that  rays  coming  from  it  are  suiBciently 
divergent  (Fig.  71)^  or  they  must  be  rendered  so  by 
passing  them  through  a  concave  lens  (Fig.  12),  before 
they  fall  upon  the  cornea. 

Fig.  70. 


Fig.  71. 


Fig.  72. 


We  may  say^  then^  that  in  myopia  the  retina  is  at 
the  conjugate  focus  of  an  object^  situated  at  a  finite 
distance.       The    accommodation    being    at    rest^  an 


MYOPIA  137 

object  situated  at  this  point  will  be  distinctly  seen ; 
further  off  it  will  be  indistinct^  nearer  it  can  still  be 
seen  clearly  by  putting  in  force  the  accommodation. 

The  greatest  distance  at  which  objects  can  be  seen 
clearly  is  called  the  far  point  (punctum  remotum), 
and  is  always  at  a  definite  distance.  The  higher  the 
myopia  the  nearer  to  the  eye  is  its  punctum  remotum 

(p.  r.). 

The  nearest  point  of  distinct  vision  is  the  punctum 
proximum  (p.  p.)^  and  is  determined  by  the  amount 
of  the  accommodation.  To  find  out  the  punctum 
proximum,  we  place  in  the  patient's  hand  the  near 
type,  and  note  the  shortest  distance  for  each  eye 
separately  at  which  the  smallest  type  can  be  read_,  or 
we  measure  it  by  the  wire  optometer  in  the  manner 
before  described.  The  amplitude  of  accommodation 
in  low  myopia  is  usually  equal  to  that  in  emme- 
tropia,  but  in  the  higher  degrees  it  becomes  consider- 
ably diminished. 

The  greatest  distance  at  which  an  object  can  be 
clearly  seen  is  the  exact  measure  of  the  myopia ;  for 
instance,  if  the  far  point  be  at  one  metre,  a  concave 
glass  of  that  strength  (  —  ID.)  w^ould  render  parallel 
rays  as  divergent  as  if  they  came  from  a  distance  of 
one  metre,  and  with  a  glass  of  this  focus  the  person 
would  be  able  to  see  distant  objects  clearly. 

Myopia  was  for  a  long  time  thought  to  be  due  to 
an  increase  in  the  convexity  of  the  cornea,  but  as 
a  matter  of  fact  the  cornea  is  usually  less  convex, 
and,  as  a  rule,  the  greater  the  myopia  the  less  the 
convexity. 


138 


THE    REFRACTION    OF    THE    EYE 


2. 


3. 


Causes  of  Myopia : 

1.  Too  great  length  of  the  antero-posterior  dia- 
meter of  the  eyeball  (axial  myopia).  This 
is  the  common  cause  of  myopia. 
Increase  of  the  index  of  refraction  of  the  lens. 
This  occasionally  occurs  in  the  development 
of  cataract. 
Conical  cornea :  this  disease  simulates  myopia 
at  its  commencement. 
It  may  therefore  be  stated  that  myopia  almost 
invariably  depends  upon  the  lengthening  of  the  visual 
axis,  accompanied  in  many  cases  by  the  formation  of 
a  posterior  staphyloma  which  further  increases  the 
antero-posterior  diameter  of  the  eyeball.  This  bulg- 
ing, when  it  occurs,  takes  place  at  the  outer  side  of 
the  optic  nerve  towards  the  macula,  and  consists 
of  an  extension  backwards  with  thinning  of  the 
sclerotic  and  choroid,  and  more  or  less  atrophy  of  the 
latter. 

So  constant  is  this  lengthening  of  the  visual  axis, 
that  from  the  number  of  dioptres  of  myopia  can 
be  calculated  the  increase  in  the  length  of  the  eye- 
ball. 

The  following  table  gives  the  calculation  up  to  10  D. 


Degree  of 

Distance  of  the  p.  r. 

Increase  in  length  of  the 

myopia. 

in  millimetres. 

myopic  eye  in  millimetres 

•5D. 

2000 

•16 

1- 

1000 

•32 

1-5 

666-6 

•49 

2- 

500 

•66 

2-5 

400 

•83 

3- 

333-3 

1^ 

MYOPIA 

1 

Degree  of 

Distance  of  the  p.  r. 

Increase  in  length  of  the 

myopia. 

in  millimetres. 

myopic  eye  m  millimetres 

3-5 

285-7 

1-19 

4- 

250 

1-37 

4-5 

222-2 

1-55 

5- 

200 

1-74 

6- 

166-6 

213 

7- 

142-8 

2-52 

8- 

125 

2-93 

9- 

111-1 

3-35 

10- 

100 

3-80 

139 


It  will  be  remembered  that  the  emmetropic  eye 
measures  in  the  antero-posterior  diameter  22*824  mm. 

Fig.  73  shows  a  section  of  a  myopic  eye,  in  which 
the  outside  measurements  were — antero-posterior 
diameter,  30^  mm.;  vertical  diameter,  25  mm.;  trans- 
verse diameter,  25  mm. 

Fig.  73. 


In  Fig.  74  the  amount  of  accommodation  is  indi- 
cated in  a  myope  of  2  D.  by  the  number  of  spaces 
through  which  the  thick  lines  pass;  thus  at  the  age 
of  thirty  the  accommodation  is  equal  to  7  D.,  and  the 
near  point  will  be  ]  1  cm. ;  the  distance  of  the  punc- 
tum  proximum  is  given  for  each  dioptre  at  the 
bottom  of  the  diagram. 


140  THE    liEEKACTlON    OF    THE    EYE 

As  the  punctum  remotum  in  myopia  is  situated  at 

a  finite  distance,  therefore^  for  the  same  amplitude  of 

accommodation,  the  punctum  proximum  is  nearer  the 

eye  in  myopia  than  in  emmetropia.     The  near  point 

Fig,  74. 

Dioptres. 


Diagram  showing  the  amount  of  accommodation  at  different 
ages  in  a  case  of  myopia  of  2  D. 

gradually  recedes  with  advancing  age  at  the  same 
rate,  whatever  the  refractive  condition  of  the  eye ;  it 
is  clear,  then,  that  the  near  point  in  myopia  Avill  be 
longer  in  reaching  that  point  (22  cm.)  at  which  pres- 
byopia is  arbitrarily  stated  to  commence  than  in 
emmetropia,  so  that  in  prescribing  glasses  for  pres- 
byopia, the  amount  of  myopia  has  to  be  deducted  from 
the  glass  which  the  emmetrope  would  require  at  any 
given  age. 

If  the  myopia  amount  to  4*5  D.,  then  the  patient 
can  never  become  presbyopic,  because  his  punctum 
remotum  is  only  22  cm.  away,  so  that  he  will  always 
be  able  to  see  at  that  distance. 

Most  people  imagine  that  those  who  do  not  require 


MYOPIA  141 

glasses  with  advancing  age  have  very  strong  eyes ; 
how  frequently  does  one  hear  the  remark,  when  in- 
inquiring  of  a  patient's  family  history,  "  Oh,  my  father 
had  excellent  sight,  he  was  able  to  read  at  sixty  without 
glasses."  This  is  proof  positive  that  he  had  myopia, 
though  probably  you  will  be  unable  to  convince  the 
patient  of  this  fact. 

In  hypermetropia  it  was  shown  that  the  power  of 
accommodation  had  to  be  used  in  excess  of  the  con- 
vergence. In  myopia  we  have  the  opposite  defect, 
the  patient  having  to  converge  in  excess  of  his  accom- 
modation ;  thus  if  he  be  myopic  4  D,,  his  far  point 
will  be  at  25  cm. ;  when  looking  at  an  object  at  this 
distance,  it  is  necessary  for  him  to  converge  to  this 
particular  point,  his  angle  of  convergence  being  4, 
while  his  accommodation  remains  passive. 

Determining^  Causes. — The  chief  factors  in  the  pro- 
duction of  myopia  are :  the  constant  use  of  the  eyes 
for  near  work,  especially  at  an  early  age,  when  these 
organs  are  developing ;  disturbances  of  nutrition  in 
the  tissues  of  the  eye,  together  in  some  cases  with  a 
peculiar  conformation  of  the  skull. 

In  a  large  majority  of  cases  myopia  is  acquired,  but 
in  a  small  proportion  of  cases  it  may  be  congenital ; 
this  latter  form  is  frequently  of  high  degree  in  early 
life,  may  occur  in  one  or  both  eyes,  and  bears  no 
relation  to  the  occupation  of  the  patient.  Though 
seldom  congenital  it  not  infrequently  happens  that 
one  or  other  of  the  parents  has  suffered  from  myopia. 

There  is  little  doubt  that  in  many  cases  there  is  an 
hereditary  tendency  to  it,  which,  transmitted  through 


142  THE    REFEACTION    OF    THE    EYE 

several  generations  under  favourable  conditions  for 
its  development,  becomes  very  decided. 

As  in  the  greater  number  of  cases  of  myopia  the 
factor  whicb  tends  to  produce  it,  is  the  proloji^ed  use 
of  the  j  eyeS:  -on  near  ob|ects,  especially  while  young ; 
we  may  set  '.down  myopia  as  one  of  the  results  of 
civilisation  and  education,  and  in  these  days  of  high 
pressure  and  competitive  examinations  it  is  constantly 
on  the  increase.  The  result  of  the  very  numerous 
statistics  that  have  been  collected,  especially  by 
German  ophthalmologists  (myopia  in  Germany  is 
exceedingly  common),  points  to  the  production  of 
myopia  in  direct  proportion  to  the  amount  of  edu- 
cation. The  amount  of  myopia  was  found  to  be 
much  greater  in  town  than  in  country  schools,  no 
doubt  because  the  general  health  was  better  amongst 
those  living  in  the  country.  Erismann  has  come  to 
the  pleasant  conclusion  that,  if  myopia  increase  in 
the  same  ratio  as  it  had  done  during  the  last  fifty 
years,  in  a  few  generations  the  whole  population 
will  have  become  "  myopic." 

The  normal  refraction  of  the  eye  in  childhood  is 
hypermetropic ;  some  few  remain  so,  a  great  number 
becoming  emmetropic  as  they  get  older,  and  a  large 
percentage  of  these  pass  on  to  myopia. 

In  proof  of  this  hereditary  tendency  to  myopia. 
Dr.  Cohn  has  summarised  the  statistics  of  various 
German  writers  on  this  subject.  In  public  shools, 
myopia  was  found  to  exist  without  predisposition 
in  8  per  cent.,  with  predisposition  in  19  per  cent. 
In     the    higher     schools     the    result    was — without 


MYOPIA  143 

predisposition  17  per  cent.,  with  predisposition  26 
per  cent. 

Residence  in  towns  is  also  conducive  to  short- 
sight  by  causing  people  to  gaze  constantly  at  near 
objects. 

The  cause  why  myopia  when  once  established  is 
very  liable  to  increase,  is  that  the  extreme  converg- 
ence, which  is  necessary  to  enable  the  patient  to 
see  at  the  limited  distance  to  which  he  is  confined, 
causes  the  weakest  part  of  the  globe  (that  part,  in 
fact,  which  is  least  supported)  to  bulge,  forming  a 
posterior  staphyloma.  In  support  of  this  method  of 
the  production  of  myopia  may  be  stated  the  well- 
known  fact,  that  people  such  as  watchmakers  and 
jewellers  who  habitually  use  a  strong  convex  lens 
before  one  eye,  and  work  at  the  focal  distance  of  that 
lens,  are  not  especially  liable  to  myopia,  proving  that 
close  work  without  convergence  does  not  tend  to  pro- 
duce it.  As  the  eyeball  becomes  elongated,  its  move- 
ments become  more  difficult,  and  the  pressure  produced 
by  the  muscles  during  prolonged  convergence  tends 
still  further  to  increase  the  myopia. 

The  stooping  position  which  so  many  myopes  take 
up  causes  an  accumulation  of  blood  in  the  eyeball 
which  tends  to  raise  the  tension  as  well  as  materially 
to  interfere  with  its  nutrition.  Hence  results  a  state 
of  congestion,  softening,  and  extension,  leading  to  a 
further  increase  of  the  myopia.  The  more  advanced 
these  changes,  the  more  difficult  is  it  for  the  myopia 
to  become  stationary. 

In    addition    to    these   two   causes,    extreme  con- 


144  THE    EEFRACTION    OF    THE    EYE 

vergence  and  the  stooping  position,  it  is  possible  tliat, 
as  a  result  of  the  constant  convergence,  the  optic 
nerves  may  be  somewhat  pulled  upon,  and  thus 
further  assist  in  producing  myopia. 

Cases  of  nebulae,  cataract,  and  other  causes  of 
imperfect  sight  in  children  may  give  rise  to  myopia 
by  causing  them  to  hold  objects  they  wish  to  see 
close  to  the  eyes. 

Symptoms. — The  patient  sees  distant  objects  badly 
and  near  objects  well.  The  eyes  look  prominent ;  the 
pupils  are  usually  large  in  young  people;  as  age 
advances  they  contract,  thus  diminishing  the  circles 
of  diffusion,  and  so  slightly  improving  vision.  Eserine 
acts  in  the  same  manner,  so  does  the  nipping  together 
of  the  eyelids,  which  is  so  characteristic  of  patients 
suffering  from  myopia,  and  to  which  the  defect  owes 
its  name.  The  acuteness  of  vision  is  frequently 
below  the  normal,  though  objects  within  the  patient's 
far  point  appear  larger  than  they  do  to  the  emme- 
trope,  the  distance  between  the  nodal  point  and 
the  retina  being  greater  in  myopia  (Fig.  75).  This, 
however,  may  be  partly  counterbalanced  by  the 
stretching  of  the  retina,  so  that,  although  the  image 
may  be  somewhat  larger,  it  may  not  cover  a  greater 
number  of  cones  than  would  be  the  case  in  an  emme- 
tropic eye. 

If  the  myopia  be  progressive,  frequent  limitations 
in  the  field  of  vision  occur,  in  the  form  of  scotomata 
due  to  patches  of  choroidal  and  retinal  atrophy. 

Besides  seeing  distant  objects  badly,  the  patient 
complains   of  pain,  fatigue,  and  intolerance  of  light, 


MYOPIA 


145 


with  a  state  of  irritation^  especially   after  using  the 
eyes  by  artificial  light.        There  may  be  hyper^emia 

Fig.  75. 


A.  The  retina  in  an  emmetropic  eye.  b.  The  retina  in  a 
myopic  eye.  c.  The  visual  angle,  n.  The  nodal  point. 
The  distance  from  n  b  is  greater  than  n  a,  and  the  image 
of  o'p  is  greater  at  b  than  at  a. 

of  the  eyes  and  lids^  spasm  of  the  accommodation 
(which  increases  the  apparent  amount  of  myopia)  pain 
in  the  eyeballs  on  pressure^  photopsia,  an  appearance 
of  convergence  due  to  the  small  size  of  the  angle  a 
(p.  202),  together  with  "  muscge  volitantes."  Muscse 
are  often  a  source  of  great  anxiety ;  the  patient  may, 
however,  be  assured  that,  although  they  cannot  be 
removed,  there  is  no  cause  for  uneasiness ;  these 
muscas  are  probably  the  remains  of  vitreous  cells, 
which,  being  situated  a  considerable  distance  in  front 
of  the  retina,  throw  shadows  on  it  and  are  projected 
outwards  as  much  larger  images  than  would  be  the 
case  in  an  emmetropic  eye  ;  they  appear  to  the  patient 
as  black  spots,  rings,  or  lines  floating  about. 

The  ciliary  muscle  is  smaller  than  in  emmetropia, 
the  circular  fibres  (which  are  so  hypertrophied  in 
hypermetropia)  being  almost  absent. 

10 


146  THE    REFRACTION    OF    THE    EYE 

The  internal  recti  muscles  often  act  badly,  so  that 
convergence  becomes  painful  and  difficult,  often  going 
on  to  divergent  strabismus. 

In  myopia  the  convergence  has  to  be  used  in 
excess  of  the  accommodation ;  some  patients  as  they 
become  myopic  learn  to  use  these  two  functions  in 
unequal  degrees,  while  others  are  unable  thus  to  dis- 
sociate them;  so  that  on  looking  at  an  object  situated 
at  the  myope's  far  point,  no  accommodation  and  no 
convergence  take  place,  it  becomes  necessary  then 
that  the  two  eyes  shall  make  a  conjugate  movement 
in  one  direction,  so  that  one  eye  may  receive  the 
image  of  the  object  on  its  macula,  the  other  eye, 
as  a  result  of  the  conjugate  movement  has  deviated 
outwards, — in  other  words,  divergent  strabismus  has 
occurred. 

When  the  myopia  is  of  high  degree,  the  patient 
often  uses  one  eye  only  for  reading,  then  of  course  he 
does  not  require  to  converge. 

The  refraction  diminishes  slightly  with  advancing 
age  (see  p.  128) ;  the  pupils  also  become  smaller,  thus 
cutting  off  some  of  the  patient's  circles  of  diffusion  ; 
so  that  frequently  a  marked  improvement  takes  place 
in  the  vision  of  myopes  as  they  get  older. 

Ophthalmoscopic  Appearances. — With  the  ophthalmo- 
scope a  crescentic-shaped  patch  of  atrophy  is  fre- 
quently seen  on  the  outer  side  of  the  optic  disc, 
embracing  it  by  its  concave  edge ;  this  is  called  the 
'^myopic  crescent." 

In  an  early  stage  the  crescent  looks  somewhat 
white,  the  large  choroidal  vessels  often  appear  more 


^^  ^^ 


Bale  <t-  Danielsson,  Ltd.,  Lith 


MYOPIA  147 

distinct  than  on  the  adjoining  parts,  while  gradually 
the  blood-vessels  disappear,  leaving  the  white  scle- 
rotic, which  shows  up  plainly  against  the  red  of  the 
fundus.  Some  remains  of  pigment  about  the  convex 
border  of  the  crescent  are  often  seen,  and  frequently 
there  is  some  thinning  of  the  choroid  beyond.  The 
retina  seems  to  participate  in  this  atrophy  much  less 
than  might  have  been  expected. 

Although  the  atrophy  usually  assumes  the  cres- 
centic  form,  as  shown  in  Fig.  1,  which  was  drawn 
from  the  fundus  of  a  young  man,  aged  twenty,  with 
a  myopia  of  4  D.,  yet  it  may  vary  much,  sometimes 
forming  a  complete  ring  round  the  optic  disc  (2),  or 
it  may  extend  outwards  (3),  the  broadest  part  being 
always  between  the  disc  and  the  macula.  Sometimes 
there  is  excavation  of  the  atrophic  part. 

The  optic  nerve  is  occasionally  displaced  somewhat 
inwards,  and  the  disc,  instead  of  being  directed  for- 
wards, looks  forwards  and  outwards,  making  it  appear 
oblong  in  shape  from  its  being  seen  obliquely  (3); 
The  retinal  vessels  that  pass  over  the  atrophied  part 
are  often  straight  in  their  course,  and  show  up  very 
clearly  against  the  white  sclerotic. 

The  formation  of  the  crescent  is  much  influenced  by 
the  amount  of  myopia.  In  slight  degrees  in  young 
people  it  is  often  absent,  but  in  cases  of  6  D.  or  more, 
at  the  age  of  twenty,  we  invariably  find  a  well-marked 
crescent. 

In  very  high  degrees  of  myopia  the  epithelial 
layer  of  the  retina  atrophies,  secondary  changes  may 
take  place  in  the  yellow  spot,   as  shown  in  Fig.  4 : 


148  THE    EEFRACTION    OF    THE    EYE 

this  mischief  may  be  due  either  to  extension  of  the 
atrophy  outward,  or  to  disease  commencing  there 
independently;  when  such  changes  take  place  they 
cause  great  impairment  of  vision.  If  the  disease  be 
progressive,  the  vitreous  becomes  disorganised,  with 
floating  opacities;  the  nutrition  of  the  lens  may 
suffer,  opacities  forming  in  it,  especially  at  the  pos- 
terior pole;  choroidal  haemorrhages  may  occur,  and 
detachment  of  the  retina  sometimes  takes  place. 

Further,  it  may  be  said  that  myopes,  owing  to  their 
defective  vision,  are  especially  liable  to  accidents. 

The  diagnosis  and  estimation  of  myopia  is  easy.  At 
the  distant  type  the  patient  requires  a  concave  glass 
to  enable  him  to  read  -J.  The  weakest  lens  with  which 
he  is  able  to  read  it  is  the  measure  of  his  myopia; 
always  remember  the  patient  is  apt  to  choose  too 
strong  a  glass  if  left  to  himself ;  to  prevent  this  and 
enable  us  to  make  an  exact  record  of  the  condition  of 
the  refraction,  by  which  we  may  judge  if  the  myopia 
is  stationary  or  progressive,  it  is  much  the  best  plan 
in  young  people  to  atropise  them  in  the  manner 
previously  described.  Great  differences  will  be  found 
in  myopes  when  testing  them  at  the  distant  type :  in 
some,  each  increase  in  the  strength  of  the  glass  causes 
a  corresponding  increase  of  vision ;  Avhile  in  others, 
with  the  same  amount  of  myopia,  but  little  improve- 
ment takes  place  until  nearly  the  full  correction  is 
reached,  when  it  suddenly  becomes  almost  normal: 
hence  it  is  not  sufficient  after  trying  two  or  three 
concave  glasses  without  any  visual  improvement,  to 
at  once  assume  the  absence  of  myopia.     On  placing 


MYOPIA  149 

the  near  type  in  his  hand,  he  will  be  found  to  be 
able  to  read  the  smallest  print,  though  at  a  shorter 
distance  than  that  for  which  it  is  marked.  The 
extreme  distance  at  which  he  is  thus  able  to  read  it 
is  his  far  point,  the  measure  of  which  is  also  a  measure 
of  his  myopia ;  this  is  a  most  useful  guide  to  us  :  for 
instance,  he  reads  No.  1  at  20  cm.  but  no  farther; 
-1^  =  5  D.,  therefore  5  D.  is  the  measure  of  the 
myopia,  and  such  a  glass  will  render  parallel  rays  so 
divergent  that  they  will  seem  to  come  from  20  cm. 
Had  he  been  able  to  read  it  at  10  cm.  only,  then 
(j_iH>.  =  10  D.)  —  10  D.  would  be  the  measure  of  the 
myopia. 

With  retinoscopy  the  shadows  move  against  the 
movements  of  the  plane  mirror  so  long  as  the  observer 
is  beyond  the  jDatient^s  far  point  (p.  87). 

With  the  ophthalmoscope,  by  the  indirect  examina- 
tion, the  disc  looks  smaller  than  in  emmetropia,  and 
becomes  larger  on  withdrawing  the  objective  further 
from  the  eye  (p.  70). 

With  the  mirror  alone  at  a  distance,  an  inverted 
magnified  image  of  the  disc  can  be  clearly  seen, 
provided  always  that  the  observer  be  not  nearer  the 
aerial  image  than  his  own  near  point  (Fig.  45).  The 
lower  the  myopia  the  greater  the  image,  because  the 
longer  is  the  distance  between  the  image  and  the 
myopic  eye.  On  moving  the  head  from  side  to  side 
the  image  of  the  disc  will  always  move  in  the  opposite 
direction,  showing  that  it  is  an  inverted  one. 

By  the  direct  method  of  examination  the  fundus  can- 
not be  clearly  seen  until  a  concave  glass  is  placed  in 


150  THE    REFEACTION    OF    THE    EYE 

front  of  the  observing  eye.  The  weakest  concave  glass 
with  which  the  details  of  the  macula  and  disc  can  be 
clearly  seen  (the  observer's  eye  being  emmetropic 
and  the  accommodation  relaxed)  is  a  measure  of  the 
myopia  (Fig.  48).  This  test  may  be  relied  upon  for 
the  lower,  but  not  for  the  higher  degrees  of  myopia. 

The  treatment  of  myopia. — The  chief  indications 
are  : 

1st.  To  prevent  the  increase  of  the  myopia. 

2nd.  To  enable  the  patient  to  see  well. 

3rd.  To  prevent  the  various  troubles  from  which 
myopes  are  so  liable  to  suif er,  as  asthenopia,  divergent 
strabismus,  etc. 

To  carry  out  the  first  of  these  indications,  strong 
convergence  and  the  stooping  position,  which  play 
so  important  a  part  in  the  production  of  myopia, 
must  be  avoided,  the  patient  being  directed  never  to 
read  in  a  train  or  carriage,  where  every  movement 
requires  a  change  in  the  accommodation  ;  he  should  not 
look  at  near  objects  for  too  long  together  :  the  natural 
tendency  for  a  myope  who  is  excluded  in  great  mea- 
sure from  seeing  distant  objects  is  to  devote  himself 
to  near  ones.  In  reading,  writing,  or  working,  he 
must  keep  35  cm.  away  from  the  book  or  paper,  use 
books  printed  in  good  bold  type,  and  not  write  too 
small,  while  the  desk  and  seat  should  be  conveniently 
arranged  so  as  to  avoid  stooping.  He  should  do  as 
little  as  possible  by  artificial  light ;  when  necessary, 
it  is  best  to  use  a  reading  lamp,  so  placed  that  it 
throws  the  light  down  upon  the  work,  leaving  the 
remainder  of  the  room  in   comparative   darkness,  so 


MYOPIA  151 

that  when  the  eyes  become  tired  they  may  be  rested 
by  turning  them  from  the  light.  The  stooping  position 
must  be  strictly  avoided,  as  it  causes  an  increased  flow 
of  blood  to  the  interior  of  the  eyeball,  and  at  the  same 
time,  by  compressing  the  veins  in  the  neck,  obstructs 
the  returning  blood,  and  so  produces  hyperaemia  with 
symptoms  of  irritation,  and  possibly  some  slight 
increase  of  tension.  When  reading  or  writing  the 
patient  should  sit  with  his  back  to  the  window,  so 
that  the  light  may  fall  on  the  book  or  paper  over 
his  left  shoulder,  the  shadow  of  his  pen  being  thus 
thrown  to  the  right,  enabling  him  to  see  plainly  the 
letters  he  is  forming. 

Attention  must  be  paid  to  the  general  health  :  iron 
internally  often  being  especially  useful,  combined  with 
regular  outdoor  exercise  and  good  nutritious  food. 

When  symptoms  of  irritation  show  themselves,  with 
a  rapid  increase  in  the  myopia,  complete  rest  must  be 
given  to  the  eyes,  and  in  no  way  can  this  be  so 
conveniently  carried  out  as  by  dropping  into  the 
eyes  a  solution  of  atropine  (gr.  j  to  3J)  three  times 
a  day,  for  some  two  or  three  weeks ;  counter-irrita- 
tion may  be  applied  to  the  temples  and  behind  the 
ears  in  the  shape  of  small  blisters,  or  by  a  solution 
of  iodine  :  no  spectacles  must  be  allowed  but  smoke- 
coloured  protectors.  Sometimes,  where  there  are 
symptoms  of  congestion  present  the  artificial  leech 
applied  to  the  temple  once  a  week  for  a  few  weeks 
does  good.  As  the  irritation  gradually  subsides, 
the  patient  may  be  allowed  to  do  a  little  reading 
daily  in    a    good   light,  the  eyes  all  the  time  being 


152  THE    REFEACTION    OF    THE    EYE 

kept  under  atropine ;  he  may  require  glasses  to 
enable  him  to  do  this.  Thus  if  he  have  myopia  of 
3  D.  he  will  not  require  them,  his  far  point  being 
at  33  cm.;  if  he  has  —  1*5  D.  he  will  require  4- 
I'o  J).,  to  enable  him  to  read  at  about  33  cm.  (+  3  D.) 
+  (—  1*5  D.)  =  +  I'D  D. ;  if  the  myopia  is  6  D.  he 
will  require  —  3  D.  to  put  back  his  far  point  from 
16  to  33  cm.   (+  3  D.)  +  (-  6  D.)  =  -  3  D. 

So  long  as  the  myopia  is  progressive  it  must  always 
be  a  source  of  anxiety  to  us. 

To  enable  the  patient  to  see  well  both  near  and 
distant  objects,  as  well  as  to  prevent  extreme  converg- 
ence, we  must  correct  the  myopia.  In  young  people 
with  good  accommodation  and  with  a  low  degree  of 
myopia  the  full  correction  may  be  well  borne,  the 
patient  wearing  such  glasses  constantly;  and  it  has 
been  observed  that  in  those  who  from  their  youth 
have  worn  their  full  correction  constantly,  for  both 
near  and  distant  objects,  the  myopia  has  in  some 
cases  remained  stationary. 

There  are  two  exceptions  to  this  general  rule  of  the 
full  correction  of  myopes  : 

1st.  Where  the  myopia  is  of  high  degree,  and 
the  acuteness  of  vision  is  reduced,  then  the  concave 
glasses  so  much  diminish  the  size  of  the  retinal 
images,  that  the  individual  is  induced  to  make  these 
images  larger  by  bringing  the  object  closer. 

2nd.  When  the  myopia  is  of  high  degree,  and 
the  patient  has  learnt,  from  long  practice,  to 
exercise  the  function  of  convergence  in  excess  of  his 
accommodation,  the   full  correction,  which  gives  him 


MYOPTA  153 

perhaps  excellent  distant  vision,  causes  him  pain 
when  used  for  near  objects.  Here  we  must  give  two 
pairs  of  spectacles,  one  for  distant  vision  and  the 
other  for  near  objects ;  the  latter  may  be  gradually 
increased  in  strength  as  the  patient  becomes  accus- 
tomed to  them,  so  that  after  a  time,  possibly  a  year 
or  so,  the  full  correction  may  be  comfortable  for 
constant  use. 

In  those  cases  where  the  myopia  is  of  high  degree, 
and  the  patient  is  unable  to  bear  the  full  correction 
for  reading,  we  find  out  the  necessary  glass  by  sub- 
tracting from  the  lens  which  gives  the  best  acuteness 
of  vision  that  glass  whose  focus  represents  the  dis- 
tance at  which  the  patient  wishes  to  read  or  work. 
Thus,  for  example,  —  9  D.  gives  the  best  distant 
vision;  the  patient  wishes  for  glasses  with  which  to 
readat33cm.(- 9D.)  +  (+3D.)  =  (_6D.);-6D. 
will  be  the  glass  required,  and  will  enable  the  patient 
to  read  at  33  cm.  without  using  his  accommoda- 
tion. 

Glasses  may  also  be  required  for  music.  When  the 
myopia  is  of  low  degree,  and  we  are  certain  that  the 
disease  is  stationary,  folders  may  be  allowed  for  dis- 
tance, no  glass  being  used  for  near  work. 

Single  glasses  are  occasionally  allowed  in  low 
degrees  of  myopia  for  looking  at  distant  objects; 
they  have  the  disadvantage  that  they  encourage  the 
patient  to  give  up  binocular  vision,  and  may  so  assist 
in  the  development  of  a  divergent  squint. 

When  muscular  asthenopia  is  present,  prisms  with 
their  bases  inwards  (which  diminish  the  necessity  for 


154  THE    REFRACTION    OF    THE    EYE 

convergence),  with  or  without  concave  glasses,  are  of 
great  value. 

When  photophobia  is  a  prominent  symptom  tinted 
spectacles  may  be  comfortable  (p.  243). 

It  is  important  to  impress  on  the  patient  that  the 
glasses  for  reading  are  not  given  to  enable  him  to 
see  better,  but  to  increase  the  distance  at  which  near 
work  can  be  done. 

When  the  myopia  has  been  estimated  under  atro- 
pine, it  is  often  necessary  to  add  on  to  the  glass  so 
found  —  '5  D.,  as  the  full  correction  under  the 
mydriatic  is  usually  this  much  weaker  than  the  cor- 
rection found  without  it,  the  reason  being  that  the 
ciliary  muscle  is  never  so  completely  relaxed  as  it  is 
by  atropine. 

I  am  of  course  aware  that  the  above  optical  treat- 
ment of  myopia  is  at  variance  with  the  teaching  of 
French  authorities. 

Landolt  considers  that  the  action  of  the  ciliary 
muscle  may  have  a  tendency  to  increase  the  myopia, 
and  therefore  states  that  myopes  should  never  wear 
glasses  which  require  the  patient  to  use  his  accommo- 
dation :  so  that  in  low  degrees  of  myopia  glasses  are 
only  allowed  for  distant  objects ;  in  medium  degrees, 
glasses  which  under-correct  the  myopia  are  given  for 
near  objects,  so  as  to  enable  the  wearer  to  see  at  a 
given  distance  without  accommodation. 

My  own  opinion  is,  that  every  case  requires  treating 
on  its  own  merits ;  very  many  myopes  wear  their  full 
correction  constantly  with  comfort,  and  if  not  with 
benefit  to  the  eyes  most  certainly  without   injury ; 


MYOPIA  iO& 

while  other  myopes  will  occasionally  be  found  who 
suffer  from  asthenopia  when  using  their  full  correction 
for  near  vision.  In  extreme  degrees  of  myopia,  and 
in  those  where  the  disease  is  increasing  rapidly,  rest 
for  the  eyes,  and  not  spectacles,  is  the  essential 
treatment. 

In  cases  of  high  myopia  (over  15  D.)  the  lens  may 
be  removed  by  a  needle  operation  followed  by  curet- 
ting, and  thus  the  eye  may  be  brought  nearly  to  the 
point  of  emmetropia,  the  patient  getting  good  distant 
vision  without  glasses.  Many  of  these  cases  have 
given  most  gratifying  results,  while  in  others  com- 
plications have  arisen  leading  later  to  detachment 
of  the  retina  or  other  troubles.  Usually  only  one  eye 
is  operated  upon. 

See  Cases  24,  31,  and  32,  pp.  247  and  255. 


156  THE    REFRACTION    OP    THE    EYE 


CHAPTER  VIII 

ASTIGMATISM    AND    ANISOMETROPIA 

Astigmatism  (^ A,  priv ;  aTL^jia,  a  point) . 

Hitherto  we  have  seen  that  the  cornea  usually 
takes  but  little  part  in  the  defects  we  have  been  con- 
sidering. It  has  been  shown  that  hypermetropia  is 
almost  invariably  due  to  the  eyeball  being  too  short, 
and  myopia  to  its  being  too  long.  We  now  come  to 
a  defect  in  which  the  curvature  of  the  cornea  plays  a 
very  important  part,  with  or  without  some  decrease 
or  increase  (from  the  emmetropic  standard)  in  the 
antero-posterior  diameter  of  the  eyeball;  I  refer,  of 
course,  to  astigmatism,  which  is  the  commonest  of 
all  the  refractive  errors,  few  cases  of  hypermetropia 
being  entirely  free  from  it,  and  still  fewer  cases  of 
myopia. 

Astigmatism  may  be  defined  as  that  state  in  which 
the  refraction  of  the  several  meridians  of  the  same 
eye  is  different  :  for  instance,  the  vertical  meridian 
may  be  emmetropic,  the  horizontal  hypermetropic. 

Astigmatism  is  usually  congenital,  but  may  be 
acquired;  frequently  there  is  some  hereditary  ten- 
dency. 


ASTIGMATISM  157 

Astigmatism  was  first  discovered  by  Thomas  Young 
in  179oj  who  was  himself  astigmatic. 

Astigmatism  is  divided  into  two  chief  varie- 
ties : 

1.  Irregular. 

2.  Regular. 

Irregular  astigmatism  consists  in  a  difference  of 
refraction  in  the  different  parts  of  the  same  meridian, 
and  may  be  further  subdivided  into  normal  and 
abnormal,  (a)  Normal  irregular  astigmatism  is  due 
in  great  measure  to  irregularities  in  the  refracting 
power  of  the  different  sectors  of  the  lens;  it  causes  a 
luminous  point  to  appear  stellate,  as  in  the  case  of  a 
star,  which  is,  in  reality,  round.  (b)  The  abnormal 
variety  may  arise  from  the  condition  of  the  lens  or  of 
the  cornea  :  when  the  lens  is  at  fault,  it  may  be  a  con- 
genital defect,  it  may  be  acquired  from  changes  taking 
place  in  the  lens  itself,  or  it  may  result  from  partial 
displacement.  The  changes  in  the  cornea  which  may 
produce  it  are,  conical  cornea,  nebulae,  and  ulcers. 
Little  can  be  done  in  the  way  of  glasses  towards 
correcting  this  form  of  astigmatism,  though  improve- 
ment of  vision  sometimes  occurs  when  stenopaic  spec- 
tacles are  worn,  the  opening  being  made  to  suit  the 
peculiarity  of  each  case. 

We  now  pass  on  to  the  much  more  common  variety, 
which  can  frequently  be  exactly  corrected  by  the  help 
of  cylindrical  lenses. 

Regular  astigmatism  is  due  to  the  curvature  of  the 
cornea  being  different  in  the  two  meridians,  that  of 
maximum  and  minimum  refraction ;  these  are  called 


158  THE    REFRACTION    OF    THE    EYE 

the  chief  meridians,  and  are  always  at  right  angles  to 
each  other. 

In  the  normal  eye  the  cornea  is  the  segment  of  an 
ellipsoid  and  not  of  a  sphere,  so  that  there  is  a  slight 
difference  in  the  refraction  of  the  two  chief  meridians, 
the  focus  of  the  vertical  meridian  being  slightly 
shorter  than  that  of  the  horizontal. 

This  can  easily  be  proved  by  looking  at  a  card  on 
which  is  drawn  two  lines  crossing  each  other  at  right 
angles;  the  card  is  held  close  to  the  eye  and  gradually 
made  to  recede ;  both  lines  cannot  be  seen  at  the 
same  time  with  equal  clearness,  the  horizontal  being 
seen  clearly  at  a  shorter  distance  than  the  vertical 
line.  So  long,  however,  as  the  acuteness  of  vision  is 
not  impaired  it  goes  by  the  name  of  normal  astig- 
matism, or  regular  astigmatism  of  the  normal  eye. 

Parallel  rays  passing  through  a  convex  spherical 
glass  come  to  a  focus  at  a  point.  If  the  cone  of  light 
thus  formed  be  divided  perpendicular  to  its  axis,  at 
any  point  between  the  lens  and  its  focus,  or  beyond 
the  focus  after  the  rays  have  crossed  and  are  diverg- 
ing, a  circle  is  formed.  In  astigmatism  the  case  is 
diiferent :  if  parallel  rays  pass  through  a  convex  lens 
which  is  more  curved  in  the  vertical  than  in  the 
horizontal  meridian,  those  rays  which  pass  through 
the  vertical  meridian  come  to  a  focus  sooner  than 
those  which  pass  through  the  horizontal;  and  the 
resulting  cone,  instead  of  being  circular  as  in  the  pre- 
vious case,  will  be  more  or  less  of  an  oval,  forming  a 
circle  only  at  one  point  (4,  Figs.  76  and  77).  Let  us 
now  divide  this  cone  at  different  points  at  right  angles 


ASTIGMATISM 


159 


to   its   axis,  and   notice   the   shape  of    the    diffusion 
patches  thus  produced. 

At  1,  an  oblate  oval  is  formed ;  at  2,  a  horizontal 
straight  line,  the  rays  passing  through  the  vertical 
meridian  having  come  to  a  focus ;  at  3,  4,  5,  the  rays 

Fig.  7G. 


Fig.  77. 


Section  of  cone  of  light  at  1,  2,  S,  4,  5,  6,  7,  Fig.  76. 


passing  through  the  vertical  meridian  have  crossed 
and  are  diverging,  and  the  rays  passing  through  the 
horizontal  meridian  are  approaching ;  at  4  a  circle  is 
formed ;  at  6  a  vertical  straight  line,  the  rays  passing 
through  the  horizontal  meridian  have  met,  while  those 
passing  through  the  vertical  meridian  are  still  diverg- 
ing ;  a  large  prolate  ellipse  is  formed  at  7. 

The  space  between  h  and  v,  h  being  the  point  at 


160  THE    REFRACTION    OF    THE    EYE 

which  the  rays  passing  through  the  horizontal  meri- 
dian focus^  and  v  the  point  at  which  the  rays  passing- 
through  the  vertical  meridian  meet,  is  called  the 
interval  of  Sturm  (i,  Fig.  78). 

Regular  astigmatism  was  at  one  time  thought  to 
be  due  to  defects  in  the  curvature  of  the  lens,  but  it 
has  since  been  proved  to  depend  almost  entirely  on 

Fig.  78. 


asymmetry  of  the  cornea.  The  lens  may,  however, 
influence  it  in  two  ways : — 1st.  Its  two  chief  meri- 
dians may  not  correspond  to  those  of  the  cornea. 
2nd.  Owing  to  the  position  of  the  eye  the  lens  may 
be  situated  obliquely. 

It  has  been  experimentally  proved  that  slight 
amounts  of  corneal  astigmatism  may  be  corrected  or 
disguised  by  the  unequal  contraction  of  the  ciliary 
muscle  (one  segment  of  the  muscle  acting  Avhile  the 
rest  of  the  circle  remains  passive)  ;  the  curvature  of 
the  lens  is  thus  increased  in  the  direction  of  the 
ciliary  contraction  only. 

In  astigmatism  the  vertical  meridian  of  the  cornea 
has  usually  the  maximum,  and  the  horizontal  meri- 
dian the  minimum  of  curvature,  corresponding  to  the 
astigmatism  of  the  normal    eye,  when  this  is  so,  we 


ASTIGMATISM 


161 


speak  of  it  as  astigmatism  according  to  the  rule.  To 
tliis^  liowever,  there  are  numerous  exceptions.  Thus 
the  chief  meridians  may  occupy  an  intermediate 
position,  or  the  vertical  meridian  may  have  the  mini- 
mum, and  the  horizontal  the  maximum  of  curvature, 
then  Ave  have  astigmatism  against  the  rule.  What- 
ever the  direction  of  the  two  chief  meridians,  they 
are  always  at  right  angles  to  each  other. 

There  are  five  varieties  of  regular  astigmatism  : 

1.  Simple  hypermetropic  astigmatism. 

2.  Compound  hypermetropic  astigmatism. 

3.  Simple  myopic  astigmatism. 

4.  Compound  myopic  astigmatism. 

5.  Mixed  astigmatism. 

In  the  first  variety,  one  set  of  rays  (we  will  assume 

the  vertical,  v)  have  come  to  a  focus  on  the  retina, 

while  those  at  right  angles,  the  horizontal   (h),  focus 

behind  the  eye.    Thus,  instead  of  a  point,  as  in  emme- 

tropia,  a  horizontal    straight   line    is   formed  on  the 

retina,  Fig.  79. 

Fig.  79. 


In  the  second  variety,  both  sets  of  rays  focus  behind 
the  retina,  forming  an  oblate  oval  on  the  retina 
(Fig.  80). 

In  the  third  variety,  one  set  of  rays  (we  will  assume 

11 


162 


THE    REFEACTION    OF    THE    EYE 


the  vertical)  focus  in  front  of  the  retina,  the  other  set 
on  the  retina,  thus  forming  a  vertical  straight  line 
instead  of  a  point  (Fig.  81). 

In  the  fourth  variety,  both  sets  of  rays  focus  in 


Fig.  80. 


Fig.  81. 


front  of  the  retina,  forming  an  upright  oval  on  the 
retina  (Fig.  82). 

Fig.  82. 


In  the  fifth  variety,  one  set  of  rays  has  its  focus  in 
front,  and  the  other  set  behind  the  retina  (Fig.  83). 


ASTIGMATISM  163 

In  these  five  figures  the  focus  of  the  rays  which 
have  passed  through  the  vertical  meridian  has  been 


Fig. 


placed  in  front  of  the  focus  of  the  rays  which  have 
passed  through  the  horizontal  meridian ;  of  course,  it 
will  be  understood  that  the  position  of  these  two  foci 
are  frequently  reversed. 

From  what  has  been  said  it  will  easily  be  seen,  that 
when  an  astigmatic  eye  looks  at  a  spot,  it  sees  not  a 
spot,  but  a  line,  an  oval,  or  a  circle ;  hence  its  name 
(a  and  aTiyfia). 

It  is  necessary  that  it  should  be  thoroughly  under- 
stood how  the  image  of  a  line  is  formed  on  the  retina : 
the  clear  perception  of  a  line  depends  upon  the  dis- 
tinctness of  its  edge,  and  to  gain  a  clear  image  of  this 
line  it  is  necessary  that  the  rays  coming  from  a  suc- 
cession of  points  which  make  up  this  line  (they  of 
course  emerge  in  every  direction)  should  be  brought 
to  a  focus  on  the  retina,  having  passed  through  the 
cornea  at  right  angles  to  the  axis  of  the  line.  Should 
they  not  do  so  circles  of  diffusion  are  formed,  which 
overlap  each  other  and  so  render  the  edges  ill-defined. 
The  rays  which  diverge  from  the  line  parallel  with  its 


164  THE    REFEACTION    OF    THE    EYE 

axis^  overlap  each  other  on  the  retinal  image,  in- 
creasing its  clearness,  except  at  the  extremities, 
where  they  overlap  and  cause  some  slight  indistinct- 
ness. Thus  a  person  with  simple  astigmatism,  myopic 
in  the  vertical  meridian  and  emmetropic  in  the  hori- 
zontal, sees  distinctly  vertical  lines,  because  the  rays 
coming  from  the  edges  of  the  vertical  line  pass  through 
the  horizontal  or  emmetropic  meridian,  while  those 
which  come  from  the  line  parallel  with  its  axis  pass 
through  the  myopic  meridian  and  overlap  jeach  other 
without  causing  any  indistinctness  of  its  edges.  There- 
fore a  patient  with  simple  astigmatism  sees  clearly 
the  line  which  is  parallel  with  his  ametropic  meridian, 
and  indistinctly  the  line  parallel  with  his  emmetropic 
meridian. 
Causes : 

1.  Congenita!  malformation  of  the  cornea,  which 

may  in  astigmatism  of  high  degree  be  part  of 
a  general  malformation  of  the  face  and  skull. 
This  variety  of  astigmatism  usually  remains 
unchanged  throughout  life. 

2.  Operations  involving   the  cornea  or  sclerotic, 

such  as  cataract  extractions,  iridectomy,  etc. ; 
these  operations  often  cause  by  their  cicatri- 
sation a  high  degree  of  astigmatism,  which 
changes  considerably  with  time. 
Symptoms. — There   is   frequently   a   want   of   sym- 
metry about  the  patient's  head  or  face.     If  young, 
and  the  astigmatism  hypermetropic  and  of  low  degree, 
few  symptoms  may  be  present ;  usually,  however,  the 
patient  complains  of  defective  vision,  with  asthenopia. 


ASTIGMATISM  165 

especially  if  his  work  be  such  that  his  accommodation 
is  in  constant  use;  sometimes  headache  is  a  very 
marked  symptom,  either  frontal  or  occipital;  he  has 
probably  tried  all  sorts  of  spectacles,  and  can  find  none 
to  suit  him.  On  trying  him  at  the  distant  type,  his 
acuteness  of  vision  is  ahvays  below  the  normal,  the 
mixed  variety  of  astigmatism  affecting  it  most,  and 
next  the  myopic  varieties.  The  way  the  patient  reads 
the  type  may  be  an  indication  of  the  defect :  he  may 
be  able  to  read  certain  letters  better  than  others ; 
thus  he  may  decipher  some  letters  of  -^  only,  and 
yet  be  able  to  read  some  of  -^j  and  even  some  of  -|. 
We  sometimes  notice,  when  trying  the  acuteness  of 
vision,  that  the  patient  sees  much  better  if  allowed 
to  hold  his  head  on  one  side ;  by  doing  this  he  places 
his  nose  somewhat  in  the  line  of  vision  of  the  eye 
he  is  using,  and  so  cuts  off  some  of  the  rays  which 
would  otherwise  enter  his  pupils,  thus  diminishing 
his  circles  of  diffusion.  It  is  possible  that  if  his  chief 
meridians  are  oblique,  by  thus  tilting  them  he  brings 
them  to  correspond  with  the  meridians  of  the  object 
looked  at.  Whether  this  explanation  be  the  correct 
one  I  know  not,  but  we  may  generally  feel  pretty 
confident,  when  we  see  the  patient  looking  at  the 
test-type  with  his  head  on  one  side,  that  astigmatism 
is  present.  One  frequently  hears  it  said  that  images 
formed  on  the  retina  in  astigmatism  are  distorted; 
this,  however,  is  not  the  case,  as  can  readily  be  proved 
by  making  one^s  own  eye  astigmatic,  by  placing  in 
front  of  it  a  cylindrical  glass :  a  certain  amount  of 
blurring  and  indistinctness  is  produced,  but  no  actual 


166  THE    REEEACTION    OF    THE    EYE 

distortion^  tlie  distance  between  the  cornea  and  retina 
being  insufficient. 

Usually  both  eyes  are  affected^  sometimes  quite 
symmetrically.  Frequently,  however,  there  is  a  great 
difference,  one  eye  being  almost  emmetropic,  the  other 
very  astigmatic. 

In  astigmatism,  when  the  chief  meridians  of  one 
eye  are  at  right  angles  to  the  chief  meridians  of  the 
other,  binocular  may  be  much  better  than  monocular 
vision :  we  will  illustrate  this  by  a  simple  example. 
The  right  eye  we  will  assume  to  be  hypermetropic 
2  D.  in  the  vertical  meridian,  emmetropic  in  the  hori- 
zontal; the  left  emmetropic  in  the  vertical,  hyper- 
metropic in  the  horizontal  2  D.  We  know  that  the 
patient,  looking  at  the  fan  of  radiating  lines  with  the 
right  eye  only,  will  see  the  vertical  lines  distinctly, 
the  horizontal  ones  only  by  accommodating ;  with  the 
left  eye  the  horizontal  lines  will  be  clearly  seen,  the 
vertical  ones  indistinctly ;  with  the  two  eyes  all  the 
lines  will  appear  fairly  distinct,  the  image  in  one  eye 
overlapping  that  of  the  other.  We  seldom  find  a 
case  in  which  the  correction  is  so  complete  as  in  our 
example,  but  we  meet  with  cases  where  partial  cor- 
rection takes  place. 

In  my  experience  vision  is  less  impaired  when  the 
chief  meridians  are  vertical  and  horizontal  than  when 
they  are  oblique. 

As  hypermetropia  is  more  common  than  myopia, 
so  also  is  hypermetropic  astigmatism  of  more  frequent 
occurrence  than  the  myopic  variety,  though  few 
myopes  will  be  found  who  are  quite  free  from  astig- 


168  THE    REFRACTION    OF    THE    EYE 

PRAY^S   TEST   TYPES   FOR  ASTIGMATISM. 

Horizontal.  15°  30° 


45°  60°  76° 

"O  ^T^  \C^' 

90°  105°  120° 

ll'lil''  f%  T/A 

llliull  kjf  //u/j 

135°  150°  165° 

%   %  ^  ^ 


ASTIGMATISM  169 

matism.  Mixed  astigmatism  is  the  least  frequently 
met  with. 

If,  after  trying  the  patient  at  the  distant  type,  we 
are  not  satisfied  with  the  result,  though  perhaps  Ave 
have  some  improvement  with  either  convex  or  con- 
cave spheres,  w^e  may  suspect  astigmatism  and  pass 
on  to  some  of  the  special  tests  by  which  it  may  be 
diagnosed  and  estimated. 

If  astigmatism  exist,  our  first  object  must  be  to 
find  out  the  direction  of  the  two  principal  meridians, 
viz.  those  of  maximum  and  minimum  refraction. 

Most  of  the  subjective  tests  for  astigmatism  are 
based  upon  the  principles  of  the  perception  of  a  line. 
An  astigmatic  eye  looking  at  a  test  object  composed 
of  lines  radiating  from  a  centre,  and  numbered  for 
convenience  like  the  face  of  a  clock,  is  unable  to  see 
all  the  lines  equally  clearly.  The  line  seen  most  dis- 
tinctly indicates  the  direction  of  one  of  the  two  chief 
meridians  ;  the  other  chief  meridian  being  of  course 
at  right  angles  to  the  one  most  clearly  seen.  The 
fan  of  radiating  lines  now  ver}"  commonl}^  used,  as 
well  as  the  clock  face  with  movable  hand,  are 
all  convenient  test  objects.  The  striped  letters  of 
Dr.  Pray  are  useful  for  indicating  one  of  the  chief 
meridians. 

To  test  and  measure  the  astigmatism,  we  place  our 
patient  at  a  distance  of  six  metres  in  front  of  the 
clock.  Fig.  84,  covering  up  one  eye  with  a  ground- 
glass  disc.  Supposing  he  sees  plainly  the  three  lines 
from  12  to  6,  all  the  other  lines  being  more  or  less 
indistinct,  those  from  3  to  9  most  so ;  and  further,  if 


170  THE    EEFRACTION    OF    THE    EYE 

on  placing  before  the  eye  a  weak  positive  glass  we 
find  that  lines  from  12  to  6  are  blurred_,  we  know  then 

Fig.  84. 


that  the  horizontal  meridian — that  is,  the  meridian 
at  right  angles  to  the  clearly  defined  line — is  emme- 
tropic, as  well  as  being  one  of  the  principal  meridians. 
We  now  direct  him  to  look  steadily  at  the  lines  from  3 
to  9,  {.  e.  those  at  right  angles  to  the  lines  first  seen, 
and  try  what  spherical  glass  enables  him  to  see  these 
lines  distinctly  and  clearly ;  this  glass  is  the  measure 
of  the  refraction  of  the  vertical  meridian,  and  there- 
fore also  of  the  astigmatism. 

To    obtain   reliable  results,  the   eye  omist  be  tho- 
roughly under  the  influence  of  atropine. 


ASTIGMATISM  171 

Supposing  lines  from  12  to  6  be  clearly  seen,  but 
that  with  a  weak  convex  glass  they  are  blurred ;  and 
that  on  looking  at  lines  3  to  9  no  convex  glass  im 

Fig.  85. 


proves  their  clearness,  while  —ID.  renders  them 
quite  distinct,  the  case  is  one  of  simple  myopic  astig- 
matism. 

With  the  ophthalmoscope  the  astigmatism  may  also 
be  recognised.  1st.  With  the  indirect  method  we 
find  that  the  shape  of  the  disc,  instead  of  being  cir- 
cular, is  more  or  less  oval,  changing  its  shape  as  the 
objective,  which  must  be  held  exactly  perpendicular, 
is  withdrawn.  2nd.  With  the  direct  method  we  find 
that  the  disc  appears  oval,  the  long  axis  of  the  oval 
corresponding  to  the  meridian  of  greatest  refraction. 
Figs.  86  and  87  show  the  same  disc  as  seen  by  the 
direct  and  indirect  examination. 

It  is,  however,  the  diiference  in  degree  of  the  clear- 
ness of  the  retinal  vessels  that  is  to  be  taken  as  the 


172  THE    REFRACTION    OF    THE    EYE 

guide,  not  only  of  tlie  cliief  meridians^  but  also  of  the 
kind  and  amount  of  error.  To  detect  this,  assuming 
that  the  chief  meridians  are  vei;tical  and  horizontal, 
we  take  notice  first  of  the  lateral  margins  of  the  disc, 

Fig.  86.*  Fig.  87. 


Erect  image.  Inverted  image. 

and  of  a  vessel  running  in  the  vertical  direction,  and 
find  out  the  b-b'ongest  positive_,  or  the  icecikest  negative 
glass,  with  which  these  are  distinctly  seen,  using  a  re- 
fracting ophthalmoscope.  We  then  take  a  horizontal 
vessel  with  the  upper  and  lower  margins  of  the  disc, 
and  estimate  their  refraction  in  the  same  manner. 
Thus  a  vessel  gomg  upwards  is  first  taken  ;  it  is  seen 
well  with  convex  1,  the  horizontal  meridian  therefore 
is  hypermetropic  1  D.  A  horizontal  vessel  is  now 
looked  at,  and  can  be  best  seen  with  concave  1,  show- 
ing that  the  vertical  meridian  is  myopic  one  dioptre ; 

*  I  have  to  thank  Mr.  Nettleship  for  these  woodcuts  from  his 
work  on  *  Diseases  of  the  Eye.' 


ASTIGMATISM  173 

the  case  is,  therefore,  one  of  mixed  astigmatism. 
When  the  chief  meridians  are  not  vertical  and  hori- 
zontal, we  must  endeavour  to  find  a  vessel  which 
coincides  with  one  of  the  chief  meridians,  and  having 
estimated  this,  we  look  for  a  vessel  at  right  angles 
to  that  first  chosen,  and  find  out  its  refraction  in 
the  same  way;  this  gives  us  the  other  chief  meri- 
dian. 

3rd.  Retinoscopy.  This  is  the  easiest  and  most  trust- 
worthy of  all  the  objective  methods.  The  patient 
being  fully  atropised,  the  principal  axes  can  be  seen 
at  a  glance,  and  the  proper  glasses  for  correcting  the 
error  easily  found  by  anyone  who  has  taken  the 
trouble  to  familiarise  himself  with  this  method  of 
examination.  For  a  full  description  of  retinoscopy 
the  reader  must  refer  to  Chap.  V,  p.  82. 

Astigmatism  requires  for  its  correction  a  cylindrical 
glass,  and  reference  has  already  been  made  to  such  a 
lens  on  p.  32. 

This  cylindrical  glass  is  the  segment  of  a  cylinder ; 
whereas  a  spherical  glass  is  the  segment  of  a  sphere. 
The  cylinder  may  be  either  concave  or  convex,  and  is 
numbered  according  to  the  refraction  of  the  meridian 
of  greatest  curvature;  the  result  upon  rays  which  pass 
through  it  is,  that  those  which  pass  through  parallel 
to  its  axis  undergo  no  refraction ;  all  other  rays  are 
refracted,  those  most  so  which  pass  at  right  angles  to 
the  cylinder.  A  cylinder  thus  possesses  the  power  of 
exactly  neutralising  the  astigmatism. 

On  referring  back  to  Fig.  79,  which  represents  a 
case  of  simple  hypermetropic  astigmatism,  the  vertical 


174  THE    EEFRACTION    OF    THE    EYE 

meridian  being  emmetropic  and  the  horizontal  meri- 
dian hypermetropic,  it  will  be  seen  that  a  convex 
cylinder  can  be  found,  which  with  its  axis  vertical 
will  increase  the  refraction  of  rays  passing  through 
the  horizontal  meridian,  so  that  they  meet  exactly  on 
the  retina.  Suppose  the  glass  required  be  +  1  D. 
cylinder,  this  not  only  corrects,  but  is  itself  a  measure 
of  the  astigmatism.  If  a  patient  with  astigmatism  of 
1  D.  be  able  to  read  -f-^  of  the  distant  type  and  with 
the  cylinder  +  1  D.  axis  vertical  -J,  it  may  be  ex- 
pressed in  the  following  manner  :  y^2'  +  -'^  -^*  ^J'  ^^^^ 
vert.  =  f . 

Fig.  80  represents  compound  hypermetropic  astig- 
matism. We  find  out  the  refraction  of  each  chief 
meridian  by  retinoscopy  or  the  clock  face.  Assuming, 
then,  the  vertical  meridian  to  be  +1  D.,  and  the 
horizontal  +  2  D.,  if  we  place  our  positive  cylinder  + 
1  D.  with  its  axis  vertical,  we  shall  have  corrected 
the  astigmatism,  and  the  error  will  be  reduced  to  one 
of  simple  hypermetropia,  requiring  for  its  correction 
4-  1  D.  sphere.  This  combination  of  sphere  -|-  1  D. 
with  cylinder  +  1  D.  axis  vertical  is  made  in  one 
glass,  by  the  optician  grinding  upon  one  side  the 
sphere  +  1  D.  and  on  the  other  the  cylinder  +  1  D. 
The  lens  thus  formed  is  called  a  spherico-cylindrical 
lens. 

Fig.  81  represents  simple  myopic  astigmatism,  in 
which  the  vertical  meridian  is  myopic  and  the  hori- 
zontal emmetropic.  To  correct  this  error  it  is  neces- 
sary to  cause  the  rays  which  pass  through  the  vertical 
meridian  to  be  so  refracted  that  they  meet  at  instead 


ASTIGMATISM  175 

of  in  front  of  the  retina.  Here  it  is  obvious  that  a 
negative  cylinder  with  its  axis  horizontal  will  accom- 
plish this  object. 

Fig.  82  represents  compound  myopic  astigmatism. 
Both  sets  o£  rays  focus  in  front  of  the  retina,  one 
set  in  advance  of  the  other.  This  is  corrected  by 
carrying  the  focus  back  by  a  negative  sphere,  and  so 
reducing  the  case  to  one  of  simple  myopic  astig- 
matism, which  is  corrected  by  a  negative  cylinder. 
This  glass  is  called  a  concave  spherico-cylindrical 
lens. 

Fig.  83  represents  mixed  astigmatism.  One  set  of 
rays  focus  in  front  of  the  retina,  the  other  set  behind 
it.  The  difference  between  these  is  the  amount  of 
astigmatism,  and  may  be  corrected  in  three  different 
ways.  Thus  supposing  the  vertical  meridian  myopic 
1  D.,  and  the  horizontal  hypermetropic  1  D.,  the  cor- 
rection may  be  made  by  —  1  D.  cylinder  axis,  horizontal, 
which  puts  back  the  vertical  rays  so  as  to  focus  on  the 
retina,  combined  with  a  +  1  D.  cylinder  axis  vertical, 
which  brings  forward  the  horizontal  rays  to  the 
retina.  This  compound  lens  is  called  a  concavo-convex 
cylinder  or  crossed  cylinders.  There  are,  however, 
some  difficulties  in  using  this  method  of  correction ; 
the  axes  of  the  cylinders  have  to  be  arranged  with 
such  exactness,  that  the  slightest  variation  may  upset 
the  whole  result.  Besides,  it  is  difficult,  when  using 
such  a  combination  at  the  distant  type,  to  make  altera- 
tions with  the  same  facility  with  which  one  does  other 
combinations.  Moreover,  during  the  grinding,  very 
great  care  is  required  of  the  optician ;  so  that 
either  of  the  following  plans  seems  preferable  :   by 


176  THE    EEFRACTION    OF    THE    EYE 

a  concave  spherical  glass  of  1  D.,  combined  with 
a  convex  cylinder  of  2  D.  axis  vertical;  or  by  a 
+  1  D.  sphere^  combined  with  —  2D.  cylinder  axis 
horizontal. 

Treatment. — Having  found  out  by  one  of  these 
numerous  methods  the  refraction  of  the  two  chief 
meridians,  we  confirm  the  result  by  trying  the  patient 
at  the  distant  type  with  the  combination  so  found, 
making  any  slight  alterations  that  may  be  necessary. 
These  glasses  may  be  ordered  at  once,  remembering, 
when  atropine  has  been  used,  that  in  hypermetropic 
astigmatism  we  must  reduce  the  convex  sphere  about 
ID.,  while  in  the  myopic  variety  the  concave  sphere 
must  be  slightly  increased  by  about  '50  D. 

We  frequently  have  to  be  satisfied  with  glasses 
which  do  not  raise  the  vision  to  f,  and  if  such  have 
been  carefully  chosen,  we  often  find  that  after  they 
have  been  worn  for  some  time  the  vision  improves, 
due  no  doubt  to  the  retina  becoming  more  sensitive  to 
well-defined  images,  a  condition  of  things  to  which  it 
was  previously  unaccustomed. 

In  ordering  glasses  for  astigmatism,  we  must  be 
careful  to  give  the  exact  axis  of  each  cylinder ;  con- 
venient forms  may  be  had  with  the  diagram  of  a 
frame  marked  in  degrees ;  the  axis  is  indicated  by 
drawing  a  line  through  this  diagram. 

The  *  Ophthalmometer  of  Javal  and  Schiotz  is  an 
instrument  for  measuring  the  amount  of  cornea  . 
astigmatism.  Scientifically  it  may  be  of  much  value, 
as  by  it  we  are  enabled  to  separate  astigmatism  due 
to  the   cornea  from   that  due   to  the   lens;   but  the 


ASTIGMATISM  177 

price  will  prevent  its  coming  into  general  use^ 
especially  as  we  possess  so  many  other  methods  by 
which  astigmatism  may  be  estimated;  the  separa- 
tion of  the  two  forms  of  astigmatism  is  a  disadvant- 
age practically,  when  we  are  seeking  to  correct 
the  defect. 

With  the  ophthalmometer  two  objects  are  reflected 
on  to  the  cornea  of  the  observed  eye  ;  these  objects 
are  of  white  enamel,  one  quadrilateral  in  shape,  the 
other  of  the  same  size  except  that  on  one  side  it  is 
cut  out  into  five  steps :  these  two  objects  slide  on 
a  semicircular  arm,  which  rotates  round  the  tube 
through  which  the  observer  looks,  one  object  on 
either  side  of  the  tube  ;  the  observer  looking  through 
this  tube,  which  contains  a  combination  of  convex 
glasses  and  a  bi-refracting  prism,  sees  four  magnified 
images  in  a  line  on  the  cornea  under  examination. 
First  find  out  the  meridian  of  least  refraction ;  this 
we  are  able  to  do  by  finding  the  position  of  the  semi- 
circular arm,  in  which  the  two  central  images  (one 
quadrilateral,  the  other  with  steps)  are  furthest  apart. 
We  slide  the  two  objects  together  until  we  see  the 
two  central  images  on  the  observed  cornea  just 
touch,  the  lowest  step  of  the  one  with  the  side  of  the 
other ;  this,  then,  is  the  meridian  of  least  refraction, 
and  we  note  it  down  as  such.  Now  turn  the  arm  at 
right  angles  to  this  meridian,  and  notice  the  amount 
of  overlapping  of  the  tAvo  central  images ;  each  step 
in  the  one  figure  that  is  overlapped  by  the  quadri- 
lateral one  is  equal  to  one  dioptre.  Thus  if  it  over- 
lap three  steps,  there  is  a  difference  of  3  D.  between 

12 


178  THE    EEiliACTlON    OF    THE    EYE 

tlie  meridians  of  least  and  greatest  refraction ;  we 
know  this  to  be  the  meridian  of  greatest  refraction, 
because  it  is  at  right  angles  to  the  one  first  found. 

As  there  are  only  five  steps,  when  there  is  a  differ- 
ence of  5  D.  between  the  two  meridians,  the  one 
figure  will  exactly  overlap  the  other :  for  higher 
degrees  we  have  to  calculate  how  much  the  figure 
with  the  steps  projects  beyond  the  quadrilateral 
figure ;  or  we  may  place  in  the  tube  a  stronger  bi- 
refracting  prism,  then  each  step  may  be  counted  as 
two  dioptres  instead  of  one. 

Nordenson  has  obtained  some  interesting  statistics 
with  this  ophthalmometer.  He  examined  with  it 
226  school  children.  As  a  result  of  these  statistics 
he  is  of  opinion — 

1st.  That  the  correction  of  corneal  astigmatism  by 
means  of  the  lens  in  young  persons  is  the  rule. 

2nd.  That  corneal  astigmatism  amounting  to  one 
and  a  half  dioptres  is  incompatible  with  normal 
acuteness  of  vision. 

Nordenson^s  observations  agree  with  the  opinion 
expressed  by  Javal  that  astigmatism  predisposes  to 
myopia. 

Tweedy^s  Optometer  affords  an  easy  method  of 
estimating  the  refraction  in  astigmatism.  It  consists 
essentially  of  a  plate  carrying  the  figure  of,  a  dial 
marked  with  fine  dark  radiating  lines  at  angles  of  15° 
with  each  other;  the  plate  is  attached  to  a  horizontal 
bar  half  a  metre  long,  divided  into  centimetres,  on 
which  it  may  be  made  to  slide  :  at  the  proximal  end 
of  the  bar  is  a  semicircular  clip,  marked  with  degrees 


ASTIGMATISM 


179 


corresponding  to  those  on  the  dial,  and  intended  to 
hold  the  cylindrical  lens.  In  order  to  use  the  instru- 
ment properly,  the  following  instructions  must  be 
strictly  complied  with  : 

1st.  The  eye  about  to  be  examined  having  pre- 
viously been  placed  completely  under  atropine,  and 
made  artificially  myopic  to  about  4  D.  by  means  of  a 

Fig.  88. 


strong  convex  lens  placed  in  a  spectacle  frame,  and 
the  opposite  eye  excluded  by  an  opaque  disc,  the 
patient  should  sit  down  before  the  instrument,  place 
the  eye  with  the  lens  before  it  close  to  the  clip,  and 
with  the  head  erect  should  look  straight  in  front  at 
the  radiating  lines  of  the  dial. 

2nd.  The  dial  having  first  been  removed  beyond 
the  point  of  distinct  vision,  should  then  be  gradually 


180  THE    EEFRACTION    OF   THE    EYE 

approximated  along  tlie  bar,  until  at  least  one  of  the 
lines  is  clearly  and  distinctly  seen;  after  this  the  dial 
should  on  no  account  be  moved,  but  its  distance  from 
the  eye  accurately  noted. 

If  all  the  radiating  lines  come  into  view  with  equal 
clearness  at  the  same  time  there  is  but  slight  astig- 
matism ;  but  if  whilst  one  line  is  clearly  seen,  that  at 
right  angles  to  it  is  blurred,  there  is  astigmatism, 
which  may  be  corrected  by  placing  in  the  semicir- 
cular clip  a  concave  cylindrical  lens  with  its  axis 
parallel  to  the  blurred  line,  or  at  right  angles  to  that 
first  distinctly  seen. 

From  the  result  of  (2)  we  learn  {a)  the  direction  of 
the  two  principal  meridians,  of  maximum  and  mini- 
mum refraction ;  (b)  the  presence  or  absence  of 
hypermetropia  or  myopia,  and  the  degree;  (c)  the 
presence  or  absence  of  abnormal  regular  astigmatism, 
including  its  direction  and  degree,  (a)  The  meridian 
of  greatest  refraction  is  parallel  to  the  line  seen  at 
the  greatest  distance  of  distinct  vision,  while  the 
meridian  of  least  refraction  is  always  at  right  angles 
to  it.  (b)  The  presence  or  absence  of  ametropia  is 
determined  by  the  distance  at  which  the  radiating 
lines  are  clearly  seen.  If  there  be  emmetropia,  the 
lines  will  be  seen  exactly  at  the  distance  of  the  focal 
length  of  the  lens  employed  to  produce  the  artificial 
myopia  :  if  there  be  hypermetropia,  the  lines  will  be 
seen  beyond  that  point;  if  myopia,  within.  The  degree 
of  ametropia  may  be  estimated  by  the  following  cal- 
culation. The  greatest  distance  of  distinct  vision, 
minus  the  focal  length   of  the    lens,  divided  by  the 


ASTIGMATISM  181 

multiple  of  these  numbers,  equals  tlie  degree  of 
ametropia. 

(c)  If,  however,  there  be  astigmatism,  the  above 
calculation  will  give  the  refraction  for  the  meridian 
of  least  refraction  only;  the  degree  of  astigmatism 
will  be  represented  by  the  focal  length  of  the  weakest 
concave  cylinder,  which,  placed  with  its  axis  parallel 
to  the  blurred  line,  makes  this  line  as  clear  and 
distinct  as  that  first  seen.  The  whole  ametropia 
may  then  be  corrected  by  combining  the  spherical 
lens  required  for  the  correction  of  the  meridian  of 
least  refraction,  with  the  weakest  cylindrical  lens 
which  by  actual  experimentation  has  been  found 
sufficient  to  correct  the  astigmatism. 

Placido's  disc  consists  of  a  circular  sheet  of  tin  on 
which  is  painted  concentric  circles  of  black  and  white; 
it  enables  one  to  detect  the  chief  meridians  of  the 
cornea  at  a  glance.  The  patient  being  placed  with 
his  back  to  the  light  is  directed  to  look  at  the  centre 
of  the  disc,  while  the  observer,  holding  the  instru- 
ment close  to  his  own  eye  and  at  a  convenient  dis- 
tance from  the  patient^s,  looks  through  the  hole  in 
its  centre ;  he  sees  an  image  of  the  concentric  circles 
reflected  on  the  cornea  :  if  astigmatism  exist,  the  rings 
will  appear  elliptical,  with  the  long  axis  corresponding 
with  the  meridian  of  least  curvature.  Cases  of  irregular 
astigmatism  and  conical  cornea  are  easily  detected  by 
this  method. 

The  stenopaic  slit,  which  consists  of  a  metal  disc 
having  an  oblong  opening  in  it  about  2  mm.  broad, 
is  used  by  some  observers  for  working  out  cases  of 


182  THE    EEFRACTION    OF    THE    EYE 

astigmatism.  The  disc  is  placed  in  a  trial  frame  in 
front  of  the  eye  we  wish  to  examine ;  and  while  the 
patient  looks  steadily  at  the  distant  type  the  disc  is 
slowly  rotated,  so  that  the  slit  is  brought  successively 
in  front  of  each  meridian ;  the  position  in  which  the 
best  vision  is  obtained  is  noted ;  we  then  try  convex 
and  concave  spheres  in  front  of  the  slit,  to  see  if  any 
improvement  take  place.  The  slit  is  now  in  line  with 
one  of  the  chief  meridians ;  let  us  turn  the  disc  round 
90°,  so  that  the  slit  may  occupy  the  position  of  the 
other  chief  meridian,  and  find  out  what  sphere  most 
improves  vision.  Thus,  supposing  with  the  slit  in  the 
vertical  direction  the  patient  reads  -|,  while  convex 
glasses  in  front  of  the  slit  make  it  indistinct,  the 
vertical  meridian  is  emmetropic ;  and  on  turning  the 
slit  so  that  it  is  horizontal,  the  patient  reads  -fj,  but 
with  +  2  D.  sphere  the  vision  equals  |-,  the  horizontal 
meridian  is  then  hypermetropic,  and  the  case  is  there- 
fore one  of  simple  hypermetropic  astigmatism,  requir- 
ing for  its  correction  +  2  D.  cylinder  axis  vertical.  On 
looking  through  the  slit,  placed  between  the  principal 
meridians,  circles  of  diffusion  are  formed,  and  the 
object  has  the  appearance  of  being  drawn  out  in  the 
direction  of  the  slit. 

Dr.  Tempest  Anderson,  of  York,  has  invented  an 
ingenious  instrument  by  which  astigmatism  may  be 
estimated  in  a  subjective  manner;  the  image  of 
an  illuminated  radiating  screen  is  thrown  on  the 
retina,  and  is  visible  to  the  observer;  the  position 
of  the  screen  on  a  graduated  bar  shows  the 
refraction. 


ASTIGMATISM  183 

Tlie  inventor  claims  for  his  instrument  the  follow- 
ing advantages : 

1.  The  observations  and  measurements  are  made 
by  the  observer,  and  are  entirely  independent  of  the 
patient's  sensations,  though  these  may  be  used  as 
an  adjunct  if  wished. 

2.  An  image  thrown  on  the  retina  being  used  as 
an  object,  the  error  arising  from  the  vessels  or  optic 
nerve  being  before  or  behind  the  retina  is  avoided. 

3.  The  refraction  and  accommodation  of  the  ob- 
server does  not  affect  the  result.  It  is  only  necessary 
that  he  should  be  able  to  see  whether  certain  lines  are 
sharply  defined. 

A  beautiful  modification  of  this  instrument  has  been 
brought  out  by  Chambers  Inskeep  &  Co.,  and  has 
been  named  "  Stigmatometer.^' 

In  addition  to  the  methods  already  described  for 
estimating  astigmatism,  many  others  are  known. 

See  Cases  5,  6,  7,  8,  9,  10  and  11,  p.  104,  etc.;  also 
33  and  34,  p.  256. 


184  THE    EEFRACTION    OF    THE    EYE 


Anisometeopia 

Anisometropia  (a,  priv. ;  Icrog,  equal;  //tVpov^  mea- 
sure ;  w;//,  the  eye)  is  the  term  applied  to  cases  Avhich 
frequently  occur,  where  the  two  eyes  vary  in  their 
refraction.  The  defect  is  usually  congenital,  but  it 
may  be  acquired,  as  in  aphakia  or  loss  of  accommoda- 
tion in  one  eye.  Every  possible  combination  may 
exist :  one  eye  may  be  emmetropic,  the  other  myopic 
or  hypermetropic;  or  one  more  myopic,  hyperme- 
tropic, or  astigmatic  than  the  other. 

Anisometropia  may  be  met  with  under  three  chief 
forms  : 

1.  Cases  where  binocular  vision  is  present. 

2.  When  the  eyes  are  used  alternately. 

3.  One  eye  is  permanently  excluded  from  vision. 

1.  In  the  first  variety  the  difference  in  refraction  is 
usually  not  very  great ;  and  if  it  were  possible  for  the 
patient  to  accommodate  unequally  in  the  two  eyes,  he 
might  be  able  to  obtain  clear  images  on  each  retina ; 
but  it  is  probable  that  the  two  ciliary  muscles  make 
the  same  effort,  with  the  result  that  in  one  eye  the 
image  is  well  defined,  in  the  other  indistinct. 

2.  When  the  eyes  are  used  alternately,  then  one 
eye  is  usually  emmetropic  or  hypermetropic,  and  is 
employed  for  distant  vision ;  while  the  other  is  myopic 
and  is  used  for  near  work. 

3.  When  the  difference  between  the  two  eyes  is 
very  great  the  best  eye  may  be  used  exclusively,  while 
the  vision  in  the   other  is  very  bad,  and  frequently 


ANISOMETROPIA  185 

deviates  outwards  or  inwards ;  in  many  of  these  cases 
one  eye  is  emmetropic  or  slightly  hypermetropic,  the 
other  highly  myopic. 

Treatment. — When  the  difference  is  not  very  great 
(1  or  1*5  D.),  and  vision  in  both  eyes  is  good,  we  may 
give  each  eye  its  correction  for  constant  use  :  for  so 
long  as  the  eye  wliose  refraction  is  the  more  defective 
still  co-operates  in  binocular  vision,  sight  is  improved 
thereby.  Especially  is  this  full  correction  useful  in 
cases  of  myopia  with  divergent  strabismus,  the  in- 
creased stimulus  to  binocular  vision  being  sometimes 
sufficient  to  prevent  the  squint. 

Many  cases  do  not  stand  their  full  correction  for 
each  eye  with  comfort;  they  complain  of  strain,  dis- 
comfort, and  headache,  though  the  younger  the  patient 
the  less  liable  is  he  to  suffer  from  these  symptoms. 

The  asthenopia  which  often  results  from  giving 
each  eye  its  exact  correction  may  possibly  be  due  to 
the  different  prismatic  effect  which  must  result  when 
the  patient  looks  obliquely  through  his  two  glasses 
which  have  a  different  refractive  power,  and  it  has 
been  suggested  by  Mr.  W.  A.  Dixey  to  overcome  this 
difficulty  by  using  a  bifocal  lens  before  the  eye 
whose  refraction  is  more  defective.  This  can  be  done 
by  grinding  a  small  central  portion  of  the  glass — that, 
in  fact,  which  is  immediately  in  front  of  the  pupil — 
of  such  a  focus  as  to  fully  correct  the  error,  while  the 
other  part  of  the  lens  will  be  ground  of  the  same 
focus  as  the  glass  in  front  of  the  less  defective  eye. 
Thus,  to  take  an  example,  a  patient  has  4  D.  of  myopia 
in  the  right  eye  and  2  D.  in  the  left ;   for  the  right 


186  THE    EEFRACTION    OF    THE    EYE 

eye  lie  would  require  a  glass  whicli  was  —  4  D.  in  the 
centre^  —  2  D.  at  the  margin,  while  the  left  eye  would 
be  supplied  with  —  2D. 

When  one  eye  is  emmetropic  and  the  other 
myopic,  no  glass  will  probably  be  required,  the 
emmetropic  eye  being  used  for  distance,  the  myopic 
eye  for  reading,  etc.  When  the  difference  in  the 
refraction  is  greater  than  1'6  D.  we  may  have  to 
be  satisfied  with  partially  correcting  the  difference, 
and  this  result  can  only  be  arrived  at  by  trying 
each  case,  some  people  tolerating  a  much  fuller 
correction  than  others,  our  object  being  to  give  as 
near  as  possible  the  full  correction  for  each  eye. 
In  children  the  full  correction  for  each  eye  can 
frequently  be  given  even  when  the  difference  between 
them  is  very  great. 

When  binocular  vision  does  not  exist  then  often 
no  attempt  can  be  made  to  correct  the  two  eyes, 
and  we  generally  give  glasses  that  suit  the  better 
eye.  In  cases  of  aphakia,  &c.,  where  one  eye  is  used 
almost  entirely,  while  the  other,  though  defective, 
still  possesses  vision,  it  is  an  excellent  plan  to  insist 
on  the  latter  being  daily  exercised  with  a  suitable 
glass,  the  good  eye  being  at  the  same  time  covered ; 
by  this  means  the  bad  eye  is  prevented  from  becoming 
worse,  and  can  at  any  time  be  utilised  should  occasion 
require. 

See  Cases  27  and  28,  pp.  250  and  253. 


PEESBYOPIA  187 


CHAPTER  IX 

PEESBYOPIA.      Pr.  {wQtcrPvg,  old ;   wi//,  eye) 

With  advancing  age  many  changes  take  place  in 
the  eye.  The  acuteness  of  vision  becomes  less^  owing 
j^artly  to  a  slight  loss  of  transparency  in  the  media, 
and  partly  to  a  diminution  in  the  perceptive  and  con- 
ductive powers  of  the  retina  and  the  optic  nerve.  At 
the  age  of  forty  the  acuteness  of  vision  is  almost 
unaltered,  the  bottom  line  of  the  distant  type  being 
read  at  a  little  over  6  metres ;  at  fifty  it  can  still  be 
read  at  6  metres,  but  after  this  time  it  diminishes 
regularly,  so  that  by  the  eightieth  year  vision  may 
have  decreased  to  |-  or  -^■^.  In  addition  to  these 
changes,  the  accommodation  gradually  diminishes 
from  a  very  early  period,  the  near  point  slowly  but 
steadily  receding.  This  change  in  the  accommo- 
dation occurs  in  all  eyes,  whatever  their  refraction, 
and  is  due  to  an  increased  firmness  of  the  lens, 
whereby  its  elasticity  is  lessened;  and  perhaps  also 
in  some  slight  degree  to  loss  of  power  in  the  ciliary 
muscle  due  to  advancing  age.  The  lens  also 
approaches  the  cornea,  increases  in  size,  and  becomes 
somewhat  flatter.  This  failure  of  the  accommodation 
begins  as  early  as  the  tenth  year,  at  an  age  when  all 
the  functions  of  the  body  are  still  developing. 


188  THE    EEFRACTION    OF    THE    EYE 

Presbyopia  must  therefore  be  looked  upon  as  a 
physiological  condition. 

When  the  binocular  near  point  has  receded  beyond 
the  distance   at  which  we    are   accustomed  to  read 

Fig.  89. 


Diagram  showing  the  course  of  accommodation  in  an  emmetropic 
eye.  The  figures  at  the  top  of  the  diagi-am  indicate  the  age ; 
those  at  the  side  the  amount  of  accommodation  and  the  p.  p. 
in  centimetres  ;  the  oblique  line  represents  the  course  of  the 
punctum  proximum,  and  the  horizontal  line  that  of  the 
punctiim  remotum ;  the  space  between  the  two  lines  gives  the 
amplitude  of  accommodation.  From  this  diagram  we  can 
calculate  the  amplitude  of  accommodation  possessed  at  any 


and  write  with  comfort,  we  become  restricted  in  our 
work.     Bonders  has  fixed  this  point  at  22  cm. 


PEESBYOPIA  189 

Presbyopia^  therefore,  may  be  arbitrarily  stated  to 
exist  when  the  binocular  near  point  has  receded  to  22 
cm.,  and  this  occurs  usually  in  the  emmetrope  about 
the  age  of  forty-five.  Because  in  order  to  see  at  22 
cm.,  a  positive  refractive  power  of  4*5  is  necessary 
(-y^O.  =  4*5) ;  at  the  age  of  forty,  the  eye  possesses 
just  this  amount  of  refractive  power;  but  if  accom- 
modation is  less,  then  we  must  give  such  a  convex 
glass  which,  added  to  the  amplitude  of  accommoda- 
tion, brings  up  the  positive  refraction  to  4"5  D.  :  for 
example,  at  the  age  of  fifty-five,  when  the  eye  possesses 
only  1*5  D.  of  accommodation,  we  give  a  convex  glass 
of  3  D.,  because  1*5  D.  -f-  3  D.  =  4*5  D.  (see  table, 
p.  191). 

To  find  the  punctum  proximum  of  an  emmetrope, 
we  have  only  to  divide  the  number  of  dioptres  of 
accommodation  which  he  possesses  into  100  cm. 
Thus  at  twenty  there  are  10  D.  of  accommodation; 
this  would  give  us  10  cm.  as  the  near  point.  At 
forty  there  are  4'5  D.,  in  which  case  the  near  point  is 
22  cm. 

When  the  punctum  proximum  has  receded  to  22 
cm.,  the  point  at  which  it  is  convenient  to  read  is 
considerably  further  away,  since  for  sustained  vision 
only  about  half  of  the  accommodation  can  be  used. 
Thus  a  person  with  4  D.  of  accommodation  would 
have  his  near  point  at  25  cm.  with  the  maximum 
contraction  of  his  ciliary  muscle,  and  if  he  can  only 
comfortably  use  about  half  this  for  continuous  work, 
his  reading  point  would  be  50  cm. ;  this  is  too  great 
a  distance.     We  bring  back  the  near  point  by  convex 


190  THE    REFRACTION    OF    THE    EYE 

glasses,  which  is  practically  the  same  as  increasing 
the  accommodation. 

Although  we  have  said  that  only  about  one  half  of 
the  accommodation  can  be  used  for  sustained  vision, 
this  is  not  absolutely  correct :  the  amount  which  must 
be  in  reserve  varies  much  with  different  individuals ; 
thus  in  one  case  with  a  surplus  of  1  D.  much  work 
can  be  done,  whereas  in  another  a  surplus  of  3  or  4  D. 
is  necessary. 

Symptoms. — The  presbyope  sees  well  at  a  distance, 
but  has  difficulty  in  maintaining  clear  vision  for  near 
objects ;  the  chief  symptoms  are  a  feeling  of  weari- 
ness in  the  eyes  after  reading,  especially  in  the 
evenings,  small  objects  being  less  easily  seen  than 
formerly,  because,  having  to  be  held  further  from  the 
eyes,  they  subtend  a  smaller  visual  angle.  The  patient 
seeks  a  strong  light,  or  places  the  lamp  he  is  using 
between  his  eye  and  the  book;  by  doing  this  he 
causes  his  pupils  to  contract,  and  so  lessens  his  circles 
of  diffusion ;  he  avoids  small  print,  and  holds  the 
book  or  work  further  away.  These  symptoms  are 
due  to  a  recession  of  the  near  point,  and  if  asthe- 
nopia occur,  this  may  be  dependent  upon  a  disturbance 
of  the  balance  between  accommodation  and  conver- 
gence ;  the  convergence  being  the  same  for  any  given 
point,  a  much  greater  accommodative  effort  is  neces- 
sary than  was  formerly  the  case. 

The  treatment  of  presbyopia  consists  in  prescribing 
convex  spectacles  for  reading  and  near  work,  so  as  to 
bring  back  the  near  point  to  a  convenient  distance. 
The  best  reading  distance  for  a  person  with  normal 


PRESBYOPIA  191 

visual  acuity  is  from  30  to  40  cm.  ;  most  emmetropes 
will,  therefore,  require  a  convex  glass  for  near  work 
soon  after  the  age  of  forty-five  ;  we  have  only  to  re- 
member to  add  on  +  1  D.  for  every  five  years  until 
we  have  reached  +  3*5  D. 

An  emmetrope  with  good  visual  acuity  will  never  re- 
quire a  stronger  glass  than  +  3*5  D.  even  when  sixty  or 
seventy  years  of  age,  because  these  glasses  adapt  him 
for  a  distance  of  30  cm.  without  any  accommodation. 

Should  the  patient  have  defective  vision,  then  it 
may  be  necessary  for  him  to  hold  near  objects  much 
closer  to  the  eyes  than  30  cm.  in  order  that  he  may 
get  larger  retinal  images;  here  we  should  be  justified 
in  prescribing  much  stronger  glasses. 

The  following  table  gives  approximately  the 
strength  of  glasses  required  by  emmetropes  at  dif- 
ferent ages  to  bring  back  their  punctum  proximum 
to  22  cm: 


Age. 
45 

Amount  of  accommodat 
'  possessed  at  that 
age. 
3-5  D. 

ion 

The  near 
point. 
28  cm. 

Glass  required  to 

bring  back 

p.  p.  to  22  cm. 

+  1D. 

50 

2-5  D. 

40  cm. 

+  2D. 

55 

1-5  D. 

67  cm. 

+  3D. 

60 

•5D. 

200  cm. 

+  4D. 

vo 

•OD. 

infinity- 

+  4-5  D. 

It  must  be  understood  that  this  table  indicates  the 
glass  that  brings  the  p.  p.  back  to  22  cm.,  and  if  we 
wish  the  patient  to  read  at  33  or  40  cm.  these  glasses 
will  in  practice  be  found  too  strong. 

To  find  the  glass  required  in  presbyopia,  we  sub- 
tract the  glass  which  represents  the  receded  near 
point  from  the  glass  whose  focus  represents  the  point 


192  THE    EEFKACTION    OF    THE    EYE 

we  wish  to  make  the  near  point.  Thus  the  near  point 
has  receded  to  50  cm, ;  the  glass  representing  this 
point  is  +  2  D.  {-^-^  =  2).  We  wish  to  bring  the 
near  point  to  25  cm.;  this  would  be  +  4 D.  {y~  =  4) ; 
hence  +  2  D.  from  +  4  D.  gives  -f  2  D.  as  the  glass 
required. 

Although  glasses  can  be  frequently  thus  ordered 
by  a  sort  of  rule  of  thumb^  it  is  always  well  to  bear 
in  mind  that  the  definition  given  of  presbyopia  with 
reference  to  its  near  point  is  entirely  an  arbitrary  one, 
and  that  we  must  take  into  account  the  distance  at 
which  the  individual  has  been  accustomed  to  read  and 
work.  In  this  there  is  great  variety.  Many  small 
people  work  and  read  at  25  cm.,  Avhereas  very  tall 
people  may  be  uncomfortable  unless  the  book  they  are 
reading  is  35  or  40  cm.  away.  The  distance  for  which 
the  presbyope  requires  spectacles  will  also  vary  much 
according  to  the  occupation  for  which  he  requires 
them  ;  thus  a  carpenter  sixty  years  old  with  emme- 
tropia  may  require  to  work  at  his  bench,  which  may  be 
at  one  metre  away;  to  enable  him  to  see  at  this  dis- 
tance he  will  require  -\-  1  D.,  while  for  reading  at 
33  cm.  he  will  require  +  ^  !)• 

There  exists  a  popular  prejudice  against  the  use  of 
strong  glasses,  all  sorts  of  maladies  having  been  attri- 
buted to  their  use.  This  prejudice  is  quite  unfounded; 
if  the  lenses  are  too  strong  they  may  bring  the  reading 
point  inconveniently  near,  and  so  produce  asthenopia 
by  causing  the  patient  to  converge  excessively. 

Before  ordering  glasses  for  presbyopia,  always 
try  the  patient's  distant  vision,  so  that   any  hyper- 


PRESBYOPIA  193 

metropia  or  myopia  may  be  recognised.  If  hyper- 
metropia  exist,  the  amount  must  be  added  to 
the  presbyopic  glass;  if  myopia,  it  must  be  sub- 
tracted. Thus  a  patient  with  hypermetropia  requir- 
ing +  2  D.  for  its  correction,  at  the  age  of  forty-five 
will  require  +  3  D.  for  reading  (H.  2  D.  +  Pr.  1  D. 
=  -f  3D.). 

A  myope  of  1  D.  will  require  no  glass  at  the  age  of 
forty-five  (M.  1  D.  +  Pr.  1  D.  =  0).  If  the  myopia 
be  3*5  D.,  then  the  patient  can  never  require  a 
glass  for  presbyopia,  his  far  point  being  at  30  cm. 
His  near  point  may  recede  to  this  distance  when  all 
accommodation  is  lost,  but  he  will  still  be  able  to  read 
at  that  distance,  though  at  that  distance  only. 

Many  people  with  a  low  degree  of  astigmatism 
have  no  discomfort  and  see  fairly  well,  therefore  they 
never  wear  a  correction  ;  when  glasses  become  neces- 
sary for  near  work  such  persons  may  prefer  a  simple 
sphere  rather  than  a  sphere  to  which  has  been  added 
their  astigmatic  correction.  Each  case  must  be  treated 
on  its  merits. 

But  it  must  be  borne  in  mind  that  as  age  advances 
the  refraction  of  the  eye  diminishes ;  in  other 
words,  the  eye  if  emmetropic  becomes  hypermetropic 
(called  acquired  hypermetropia).  The  myopic  eye 
becomes  less  myopic,  so  that  a  real  improvement  in 
vision  takes  place.  The  hypermetropic  eye  becomes 
more  hypermetropic.  This  change  takes  place  at  a 
regular  rate  in  all  eyes ;  at  fif tj^-five  the  refraction  has 
diminished  "25  D.,  at  sixty-five  -75  D.,  at  sixty-eight 
1   D.,  and   at   eighty  as   much   as  2*5  D.     Thus  at 

13 


194  THE    EEFEACTION    OF    THE    EYE 

eighty  an  emmetrope  will  have  acquired  2*5  D.  of 
hypermetropia,  and  will  therefore  require  a  convex 
glass  -H  2'5  D.  for  distant  objects  to  be  seen  clearly. 
A  myope  of  2*5  D.  would  at  eighty  have  become 
emmetropic,  and  require  no  glass  for  distance.  A 
hypermetrope  of  2'5  D.  will  add  on  to  his  defect  2*5  D., 
and  will  require  a  +  5  D.  for  distance.  This  change 
is  due  to  sclerosis  and  enlargement  of  the  crystalline 
lens,  by  which  its  refractive  power  is  diminished. 

Dr.  Scheffler  some  years  ago  proposed  the  use  of 
orthoscopic  lenses,  that  is,  lenses  with  two  elements, 
a  sphere  and  a  prism,  so  proportioned  that  the 
amount  of  accommodation  and  convergence  should 
exactly  correspond.  Thus  in  the  case  of  a  pres- 
byope  aged  fifty,  requiring  +  2  D.  to  make  him 
read  comfortably  at  25  cm.,  it  would  be  combined 
with  a  prism  of  2  m.a.  base  inwards,  the  result  being 
that  the  patient  would  .then  have  to  put  on  2  D.  of 
his  accommodation  and  2  m.a.  of  convergence,  and 
thus  these  two  functions  would  be  used  in  equal 
degrees.  The  results,  however,  are  not  so  good  as 
might  have  been  hoped;  the  glasses  are  too  heavy, 
and  on  looking  at  a  flat  surface  some  distortion  is 
produced.  Nevertheless  cases  do  occur  in  which, 
though  the  presbyopia  is  corrected,  the  patient  after 
reading  a  short  time  complains  of  asthenopia.  Such 
cases  are  frequently  at  once  and  completely  relieved 
by  combining  with  the  spheres,  prisms  of  2°  or  3° 
with  their  bases  inwards;  or  by  having  the  lenses 
decentred,  forming  convex  prismo-spheres   (Fig.  102). 

A  lens  of  1   D.  must  be  decentred  8*7  mm.  to  pro- 


PRESBYOPIA  195 

duce  a  prismatic  effect  of  1°.     Thus  in  order  to  find 

out   the    amount    that    a    lens    should  be    decentred 

to  produce  a  given  prismatic  effect^  it  is  necessary  to 

multiply  8*7  by  the  number  of  the  prisms  we  wish  to 

use,    and    divide    the    result    by  the   number  of  the 

lens.     Thus,  to  take  an  example,  with  a  +  6  D.  we 

wish    to    produce    a    prismatic    effect    of    2°,    then 

8-7  X  2  _  17-4  _  ^  _  ^,        ,  , , 

^ —^——  2*9  mm.;  the  glass  would  reqmre 

to  be  decentred  inwards  2"9  mm. 

Care  must  be  taken  to  see  that  all  glasses  are 
properly  centred,  unless  they  have  been  ordered 
otherwise  ;  for  if  the  frames  are  too  broad  or  too 
narrow,  the  prismatic  effect  produced  is  very  apt  to 
give  rise  to  asthenopia,  by  disturbing  the  relations 
between  convergence  and  accommodation. 

In  cases  where  the  convex  glasses  have  frequently 
to  be  changed  for  stronger  ones,  "  glaucoma"  should 
be  carefully  looked  for;  and  if  any  symptoms  of  it 
appear,  no  near  work  must  be  allowed,  as  it  is 
important  to  avoid  all  possible  causes  of  tension. 

The  commencement  of  cataract  may  in  some  cases 
hasten  presbyopia,  but  it  more  frequently  produces 
myopia,  so  that  the  presbyope  requires  his  glasses 
diminished  in  strength. 

In  each  case  of  presbyopia  first  test  the  patient^s 
distant  vision,  so  as  to  detect  any  hypermetropia, 
myopia,  or  astigmatism ;  and  having  recorded  this, 
we  add  the  glass  which  he  requires  for  his  presbyopia 
and  try  him  with  the  reading  type  :  should  they  suit, 
we   direct  the  patient  to  read  with  them  for  half  an 


196  THE    EEFEACTION    OF    THE    EYE 

hour  or  so ;  if  found  satisfactory  we  order  spectacles 
of  this  strength. 

See  cases  25,  29,  30,  and  31,  p.  249,  etc. 


Paealysis   oe   the   Accommodation 

Paralysis  of  the  accommodation,  either  partial  or 
complete,  arises  from  loss  of  power  in  the  ciliarj^ 
muscle  (cycloplegia),  and  is  due  to  paralysis  of  the 
third  nerve,  or  of  that  branch  of  it  which  supplies  the 
muscle  of  accommodation  and  the  circular  fibres  of 
the  iris.  Cases  do  occasionally  occur,  though  very 
rarely,  of  paralysis  of  the  ciliary  muscle  not  involving 
the  constrictor  pupillas.  Both  eyes  may  be  affected, 
or  only  one. 

When  the  paralysis  is  confined  to  the  ciliary  muscle 
and  iris,  it  goes  by  the  name  of  ophthalmoplegia 
interna. 

Causes. — Atropine  is  the  most  common  cp.use,  but  it 
may  be  due  to  diphtheria,  rheumatism,  fever,  any 
complaint  of  a  lowering  character,  cerebral  trouble, 
syphilis,  diabetes,  or  some  reflex  irritation,  e.  g.  decayed 
teeth,  &c. ;  the  cause  may,  however,  not  be  apparent. 
When  the  whole  third  nerve  is  involved,  ptosis,  ex- 
ternal strabismus,  &c.,  occur ;  but  in  those  cases 
where  the  branch  supplying  the  ciliary  muscle  and 
the  circular  fibres  of  the  iris  is  alone  implicated,  the 
indicating  symptoms  are,  asthenopia,  dilatation  of  the 
pupil,  and  loss  of  the  power  of  accommodation, 
whereby  the  patient,  though  able  to  see  distant 
objects  well  (if  emmetropic),  is  unable  to  read  or  do 


SPASM    OF    THE    ACCOMMODATION  197 

any  near  work.  In  liypermetropia,  both  near  and 
distant  vision  will  be  impaired ;  in  myopia^  the  patient 
is  able  to  see  only  at  his  far  point.  We  try  the 
patient  at  the  distant  type,  and  if  he  is  able  to  see  f 
and  yet  is  not  able  to  read  near  type,  the  diagnosis  is 
obvious. 

Treatment  consists  in  giving  such  convex  glasses  as 
enable  him  to  read.  In  order  to  bring  the  emme- 
trope's  far  point  from  infinity  to  33  cm.,  +  3  D.  is 
required  {-^J~  =  3).  We  must  bear  in  mind  that 
by  encouraging  the  action  of  the  ciliary  muscle  we 
hasten  the  patient^ s  recovery ;  we  therefore  order  the 
weakest  convex  glasses  with  which  he  is  able  to  read, 
changing  them  for  weaker  ones  occasionally  as  the 
ciliary  muscle  gains  strength.  Sulphate  of  eserine  in 
solution,  gr.  j  to  ^j,  causes  contraction  of  the  ciliary 
muscle  as  well  as  of  the  iris,  and  temporarily  relieves 
the  symptoms.  I  think  much  good  sometimes  results 
from  its  use  once  every  other  day  for  some  weeks  ; 
the  ciliary  muscle  being  made  to  contract,  relaxing 
again  as  the  elfect  of  the  myotic  passes  off  :  some- 
times the  local  application  of  electricity  is  useful. 
Attention  must  be  paid  to  the  general  health,  iodide 
of  potassium  or  nerve-tonics  being  given  when  indi- 
cated by  the  cause. 

See  Case  26,  p.  250. 


Spasm  of  the  Accommodation 

Spasm  of  the  accommodation  may  be  of  two  kinds. 
Tonic  and  Clonic. 


198  THE    REFRACTION    OF    THE    EYE 

Clonic  spasm  occurs  only  when  tlie  eye  is  in  use, 
ceasing  as  soon  as  it  is  in  a  condition  of  repose. 

Tonic  spasm  is  more  permanent,  requiring  atropine 
or  one  of  the  other  mydriatics  for  its  relief  ;  the  ex- 
pression spasm  of  accommodation  usually  refers  to  this 
variety  of  the  disorder. 

Tonic  spasm  of  the  ciliary  muscle  may  be  occasion- 
ally met  with  in  eyes  whatever  their  refraction, 
though  most  commonly  in  cases  of  hypermetropia  and 
low  myopia ;  it  has  the  effect  of  increasing  the  re- 
fraction of  the  eye,  and  is  found  most  frequently  in 
children. 

Causes. — It  may  occur  as  a  result  of  uncorrected 
ametropia,  or  in  emmetropia  from  overwork,  espe- 
cially when  such  work  has  been  done  in  a  bad  light ; 
or  it  may  follow  contusion  of  the  eyeball,  and  sometimes 
it  occurs  with  cyclitis. 

Symptoms. — It  usually  affects  both  eyes,  giving  rise 
to  symptoms  of  asthenopia  with  a  feeling  of  con- 
striction and  discomfort  in  the  eyes  themselves ;  there 
may  be  an  increased  secretion  of  tears  with  or  without 
blepharospasm ;  the  acuteness  of  vision  may  be 
diminished  and  is  very  variable,  while  the  size  of  the 
pupil  usually  remains  unaffected.  In  emmetropia  we 
may  get  symptoms  of  myopia,  owing  to  the  parallel 
rays  coming  to  a  focus  in  front  of  the  retina.  In 
hypermetropia  the  symptoms  may  also  simulate 
myopia,  and  for  this  we  should  always  be  on  our 
guard.  I  have  on  several  occasions  seen  liyper- 
metropes  going  about  wearing  concave  glasses  to 
correct    their    supposed     short-sightedness.         Some 


SPASM    OV    THE   ACCOMMODATION  199 

time  ago  I  saw  a  young  man  who  had  worn  —  7  D. 
constantly  for  years,  though  his  refraction  was  really 
emmetropic.  In  myopia  the  real  defect  is  apparently 
increased  by  the  spasm,  and  we  might  be  in  danger  of 
ordering  too  strong  concave  glasses,  &c.  For  these 
reasons  the  systematic  use  of  atropine  in  young  people 
(whereby  one  is  enabled  to  estimate  and  record  the 
exact  state  of  the  refraction)  cannot  be  too  strongly 
insisted  upon.  The  treatment,  where  spasm  of  the 
ciliary  muscle  is  suspected,  is  to  drop  into  the  eyes 
three  times  a  day  a  solution  of  sulphate  of  atropine, 
grs.  iv  to  5J;  foi'  two  or  three  weeks ;  this  quickly 
relieves  the  spasm,  and  gives  the  eyes  complete  rest : 
any  ametropia  that  may  exist  must  be  corrected,  and 
the  patient's  general  health  attended  to,  tonics  being 
administered  if  necessary. 

A  few  cases  of  acute  spasm  of  the  accommodation 
have  been  recorded  which  resisted  the  treatment  by 
atropine ;  the  spasm,  though  relaxed  by  this  means, 
returned  as  soon  as  the  atropine  was  discontinued. 

See  Case  1,  p.  102. 


200  THE    KEFKACTION    OF    THK    EYR 


CHAPTER  X 

Strabismus   [cTTpirpw,  I  turn  aside] 

Strabismus  exists  when  tliere  is  a  deviation  in  the 
direction  of  the  eyes^  so  that  the  visual  axes  are  not 
directed  to  the  same  object. 

Strabismus  may  be  divided  into  two  classes — 
Concomitant. 
Paralytic. 

Concomitant  strabismus  is  a  frequent  result  and 
complication  of  refractive  errors_,  and  has  already  been 
referred  to  on  pages  124  and  146. 

The  deviation  of  one  eye  from  its  correct  position 
is  the  result  of  disturbed  muscular  equilibrium^  and 
may  be  due  to  — 

1.  Defective  anatomical  conditions,  or 

2.  Abnormal  innervation  causing  contraction  of 

the  muscles  not  in  accordance  with  the  re- 
quirements of  binocular  vision.  Most  cases 
of  concomitant  strabismus  are  due  to  this 
second  cause. 

The  points  to  note  when  a  case  of  strabismus 
presents  itself  are — 

1.  Is  the  strabismus  real  or  only  apparent  ? 


STEABISMUS  201 

2.  If  real,  to  which  variety  does  it  belong  ? 

3.  Which  is  the  deviating  eye  ? 

4.  In  which  direction  is  the  deviation  ? 

5.  What  is  the  degree  of  the  deviation  ? 

6.  What  is  the  cause  of  the  strabismus  ? 

The  first  of  these  questions  may  seem  unnecessary, 
but  it  is  not  always  easy  to  say  if  squint  is  really 
present  or  not,  because  one  judges  of  the  direction  of 
the  eyes  by  the  position  of  the  centres  of  the  corneae, 
or  rather  by  the  optic  axes,  and  these  may  diverge 
slightly  while  the  visual  axes  are  really  parallel.  This 
requires  some  explanation. 

The  visual  axis  is  the  line  passing  from  the  macula 
through  the  nodal  point  to  the  object  looked  at,  shown 
in  the  following  figures  as  v  m. 

Fig.  90. 


The  optic  axis  is  the  line  passing  through  the  nodal 
point  and  the  centre  of  the  cornea  to  the  inner  side  of 
the  macula,  o  h  in  figures. 

It  will  thus  be  seen  that  these  two  axes  form  an 
angle  at  the  nodal  point  which  in  emmetropia  amounts 
usually  to  5°^  (Fig.  90). 

*  This  angle  is  by  some  authors  called  the  angle  y. 


202  THE    EEFEACTION    OF    THE    EYE 

This  is  called  the  angle  a,  and  when  thus  formed 
by  the  crossing  of  the  visual  and  optic  axes  it  is  said 
to  be  ijositive. 

In    hypermetropia  (Fig.  91)  the  angle  a  increases 

Fig.  91. 


with  the  degree  of  hypermetropia^  and  if  it  be  high 
may  attain  7°,  8°,  or  even  more ;  this  large  angle  gives 
to  the  eye  an  appearance  of  divergence. 


Fig.  92. 


M.  The  maciila.  n.  The  nodal  point,  b.  Optic  nerv^e.  v.  The 
object.  V  M.  The  visual  axis,  o  h.  The  optic  axis.  a. 
The  angle  alpha  formed  between  the  visual  and  optic  axes. 
c.  The  centre  of  rotation  of  the  eyeball  situated  on  the 
optic  axis.  y.  The  angle  gamma  (Fig.  90)  is  formed  at 
the  centre  of  rotation  of  the  eyeball,  by  the  optic  axis  and 
a  line  drawn  from  the  centre  to  the  object  looked  at. 

In  myopia  (Fig.  92)  the  angle  a  decreases^  and  in 


STEABISMUS 


203 


high  myopia  the  visual  axis  may  approach  the  optic 
axis,  so  that  the  angle  a  is  very  small,  or  it  may 
coincide  with  it  when  no  angle  is  formed ;  or  even  be 
altogether  on  the  outer  side  o£  it,  when  the  angle  is 
said  to  be  negative.  This  small  angle  a  gives  to  the 
eyes  an  appearance  of  convergence."^ 

In  order  to  find  out  the  variety  to  which  our  case 
of  strabismus  belongs,  as  well  as  to  decide  which  is 
the  deviating  eye,  we  direct  the  patient  to  look  at  an 
object  held  about  a  metre  in  front  of  him,  then 
gradually  bring  this  object  nearer  to  him,  so  as  to  call 
into  action  the  accommodation :  if  both  visual  axes 
continue  to  be  directed  steadily  towards  the  object 
as  it  is  made  to  approach  the  eyes,  the  case  is  one 
of  apparent  strabismus ;  but  if  one  eye  fix  the  object, 
while  the  other,  after  following  it  up  to  a  certain  dis- 
tance, suddenly  deviates  inwards  or  outwards,  the 
condition  is  spoken  of  as  concomitant  strabismus 
(convergent  or  divergent)  ;  or  both  eyes  may  follow 
the  object  up  to  a  certain  point,  when  one  eye  stops, 
after  making  a  few  jerking  oscillating  movements; 
it  then  belongs  to  the  paralytic  variety  of  strabismus. 

Again,  direct  the  patient  to  look  at  some  object  held 
midway  between  the  two  eyes  and  about  a  metre 
away;  if  the  right  eye  fix  the  object  while  the  left 
deviates  inward,  we  mark  upon  the  edge  of  the  lower 
lids  with  a  pen  the  position  of  the  external  margin  of 

*  Another  angle  sometimes  mentioned  is  the  angle  y,  which  is 
the  angle  formed  at  the  centre  of  rotation  of  the  eye  by  the  optic 
axis  and  a  line  drawn  from  this  centre  to  the  object  looked  at, 
shown  in  Fig.  90. 


204 


THE    REFRACTION    OP    THE    EYE 


the   cornea  in  eacli  eye,  s   and  p  :    on   covering    the 
right  eye  with  a  card,  the  left  will  at  once  make   a 


Fig.  93. 


P      D 
L 


The  two  upper  diagrams,  r  and  l,  show  the  primary  position 
of  the  eyes,  the  right  being  the  fixing  eye,  while  the  left 
is  deviating  inwards.  On  covering  the  right  eye  with  a 
card,  as  shown  by  the  dotted  lines,  the  left  eye  fixes  while 
the  right  deviates  inwards,  p  d  therefore  indicates  the 
primary  deviation,  s  d  the  secondary  deviation. 

movement  outwards  to  fix  the  object,  and  we  make  a 
second  ink  mark,  d,  on  the  lids  corresponding  with 
the  outer  edges  of  the  cornea  in  this  position. 

The  distance  p  d  gives  us  the  amount  of  lyrlmary 
deviation-,  it  may  further  be  seen  that  on  covering  the 
right  eye  so  as  to  cause  the  left  eye  to  fix  the  object 
looked  at,  the  right  eye  has  made  a  movement  inwards 
behind  the  screen,  a  s'econdarij  deviation  has  taken 
place,  this  is  recorded  l)y  an  ink  mark  on  the  right 


STRABISMUS  205 

lower  lid  at  d  ;  we  have  thus  found  the  linear  measure- 
ment of  the  secondary  deviation,  s  d. 

The  primary  deviation  (p  d)  will  be  found  to  equal 
the  secondary  deviation  (s  d),  a  characteristic  of 
concomitant  squint. 

In  paralytic  squint  the  secondary  deviation  is  always 
greater  than  the  primary. 

This  variety  of  squint  is  known  as  concomitant, 
because  the  squinting  eye  follows  the  fixing  one  in  all 
its  movements,  the  amount  of  squint  is  the  same  in 
whichever  direction  the  eyes  are  turned ;  therefore 
the  range  of  movement  in  concomitant  squint  is  as 
great  as  in  cases  where  no  squint  exists;  it  is  simply 
displaced.  In  the  paralytic  variety,  the  movements 
of  the  squinting  eye  are  usually  much  curtailed ;  this 
we  detect  by  holding  up  the  finger  about  50  cm.  in 
front  of  the  patient,  and  directing  him,  while  keeping 
the  head  still,  to  follow  the  movements  of  the  finger, 
which  is  moved  to  either  side,  then  up  and  down. 

So  that  in  concomitant  strabismus  the  squinting 
eye  will  almost  exactly  accompany  the  other,  the 
visual  lines  being  at  the  same  angle,  except  perhaps 
in  the  extreme  periphery,  whereas  in  paralytic  squint 
one  eye  will  stop  at  a  certain  point,  while  the  other 
eye  continues  to  follow  the  finger. 

Concomitant  squint  is  characterised  by  the  fact 
that  the  primary  and  secondary  deviations  are  equal ; 
in  paralytic  strabismus  the  secondary  deviation  ex- 
ceeds the  primary ;  in  paralytic  squint  the  diplopia 
is  the  most  prominent  subjective  symptom,  while  in 
the  concomitant  variety  it  is  seldom  complained  of. 


206 


THE    EEFRACTION    OF    THE    EYE 


When  either  eye  fixes  indifferently,  the  vision 
being  equally  good  in  both,  the  strabismus  is  alternat- 
ing ;  when  the  same  eye  always  squints,  it  is  mono- 
lateral  or  constant.  The  vision  in  the  squinting  eye  is 
often  below  that  in  the  fixing  one. 

Periodic  is  the  name  applied  to  the  squint  when  it 
only  comes  on  occasionally,  as  after  looking  for  some 
time  at  near  objects.  With  judicious  treatment  this 
variety  can  be  cured  without  operation ;  if  neglected 
it  generally  passes  on  into  one  of  the  constant 
forms. 

There  are  several  ways  by  which  the  amount  of  the 
deviation  may  be  estimated.  Thus  we  may  record  it 
in  the  form  of  a  diagram  (Fig.  93). 

The  strabismometer  (Fig.  94)  consists  of  a  handle 

Fig.  94. 


Strabismometer. 


STEABISMUS  207 

supporting  a  small  ivory  plate,  shaped  to  the  lower 
lid,  and  having  on  it  a  scale  by  which  Ave  measure  the 
amount  of  deviation  of  the  centre  of  the  pupil.  This 
is  an  easy  method  of  measuring  the  strabismus,  but  is 
not  to  be  depended  upon. 

The  measurement  of  the  angle  of  the  strabismus  is 
the  best  method  of  recording  the  amount  of  squint. 
The  angle  of  the  strabismus  may  be  defined  as  that 
angle  which  the  visual  axis  makes  with  the  direction 
it  should  have  in  a  normal  state. 

For  this  measurement  we  require  a  perimeter,  in 
front  of  which  we  seat  the  patient,  with  the  quadrant 
placed  according  to  the  kind  of  squint  we  are  about 
to  measure ;  if  it  be  convergent  or  divergent,  then 
the  quadrant  is  placed  horizontally.  The  patient 
being  seated  so  that  his  deviating  eye  is  in  the  centre 
of  the  instrument,  we  direct  him  to  fix  with  both  eyes 
some  distant  object  (o.  Fig.  95)  placed  in  a  line  with 
the  centre  of  the  perimeter ;  a  lighted  candle  is  moved 
gradually  along  the  inside  of  the  quadrant  from  the 
centre  of  the  instrument  outwards ;  the  observer, 
following  the  movement  of  the  candle  with  his  head, 
stops  as  soon  as  the  reflection  of  the  candle  on  the 
cornea  of  the  squinting  eye  occupies  the  centre  of  its 
pupil,  this  gives  the  direction  of  the  optic  axis ;  what 
we  really  wanted  was  the  direction  of  the  visual  axis, 
but  for  all  practical  purposes  the  former  is  sufficient. 
The  degree  is  read  off  the  quadrant  at  the  point  where 
the  candle  was  stopped,  and  this  result  recorded. 
The  angle  of  deviation  for  near  vision  should  next  be 
taken;  this  is  done  by  requesting  the  patient  to  look 


208  THE    EEFEACTION    OF    THE    EYE 

at  the  centre   of  the  perimeter,  proceeding  with  the 
candle  as  before,  and  recording  the  result. 

Fig.  95.* 


Concomitant  strabismasis  intimately  connected  with 
hypermetropia  and  myopia  ;  it  may  be — 
Convergent 

or 
Divergent. 

Convergent  Concomitant  Strabismus. — On  looking  at 
any  object,  one  eye  only  is  directed  to  it ;  the  other, 
as  the  name  implies,  turns  inward  so  that  the  angle 

*  In  the  above  diagram,  o  is  intended  to  represent  a  distant 
object ;  it  is  placed  near  the  perimeter  in  order  to  take  up  less 
room. 


CONVERGENT    STRABISMUS  209 

of  convergence  is  much  greater  in  the  deviating  than 
in  the  fixing  eye. 

This  variety  of  squint  is  usually  due  to  hyper- 
metropia^  at  least  80  per  cent,  of  the  cases  are  due  to 
this  cause  ;  its  method  of  production  depends  upon 
the  intimate  connection  that  exists  between  accom- 
modation and  convergence. 

The  convergence  is  most  marked  when  looking  at 
near  objects ;  sometimes  there  may  be  no  squint  when 
distant  objects  are  viewed. 

A  person  who  is  hypermetropic  requires  to  use 
some  of  his  accommodation  for  distance;  for  near 
objects  he  must,  of  course,  use  still  more,  and  for 
every  increase  in  the  accommodation  there  is  a  desire 
for  an  equal  increase  in  the  degree  of  convergence. 
Thus  an  emmetropic  individual,  accommodating  for 
an  object  at  30  cm.,  would  at  the  same  time  converge 
for  that  particular  point. 

If  the  person  were  hypermetropic  to  the  extent  of 
4  D.,  and  supposing  the  amplitude  of  his  accommo- 
dation amounted  to  8  D. ;  then  he  would  require  to 
use  half  this  (4  D.)  to  enable  him  to  bring  parallel 
rays  to  a  focus  on  the  retina ;  and  he  would  have  the 
tendency  at  the  same  time  to  use  4  metre-angles  of 
convergence.  Thus  for  distant  objects  he  would  have 
an  inclination  to  converge,  his  internal  recti  acting ; 
and  it  is  only  by  the  increased  tension  of  the  external 
recti,  called  into  action  by  the  desire  which  all  eyes 
possess  for  singleness  of  vision,  that  convergence  is 
prevented.  The  more  we  accommodate  the  greater 
is  the  stim^ulus  to  converge,  so  that  on  looking  at  near 

14 


210  THE    EEFEACTION    OF    THE    EYE 

objects — wliicli  necessitates  an  increase  of  the  accom- 
modation— an  increased  tendency  to  convergence  is 
produced. 

Fig.  96. 


R.  Right  eye  directed  to  object  o.     l.  Left  eye  deviating 
inwards,     m.  Macnla. 


Now,  if  the  hypermetropia  be  of  such  a  degree 
that  for  any  given  point  of  convergence  it  exceeds 
the  positive  part  of  the  relative  accommodation  (Fig. 
34,  p.  50),  one  of  two  things  must  occur;  either  the 
patient  must  see  indistinctly  by  not  accommodating 
sufficiently,  or  one  eye  must  be  allowecl  to  converge. 


CONVERGENT    STRABISMUS  211 

Some  patients  will  prefer  binocular  indistinct  vision, 
others,  monocular  clear  vision  with  squint. 

One  occasionally  finds  an  individual  who  can  thus 
choose  which  he  will  do ;  we  are  trying  his  acuteness 
of  vision  at  the  distant  type  perhaps;  he  stops  at 
some  place,  we  will  suppose  -f^,  and  says  that  he  is 
unable  to  read  the  next  two  lines  unless  he  squint. 
The  accommodation  necessary  to  read  -|  makes  a 
heavier  call  on  the  convergence  than  can  be  borne ; 
such  a  case  forms  a  good  illustration  of  the  manner 
in  which  convergent  strabismus  is  produced  in  a 
hypermetrope. 

Hence,  if  the  impulse  to  see  distinctly  is  greater 
than  the  desire  to  retain  binocular  vision,  one  eye 
yields,  and  squint  occurs ;  at  first  diplopia  follows 
the  convergence,  and  is  always  in  the  opposite  direc- 
tion to  the  deviation.  Possibly  the  convergence  of 
the  deviating  eye  is  increased  by  the  desire  that  the 
weaker  image  may  be  made  still  weaker,  by  falling 
on  a  more  peripheral  part  of  the  retina. 

In  most  cases  squint  begins  before  binocular  vision 
is  acquired,  so  that  the  fusion  faculty  is  imperfectly 
developed,  the  squinting  eye  is  but  little  used,  and 
the  eye  quickly  becomes  amblyopic. 

There  are  two  varieties  of  amblyopia  met  with  in 
squint — a  rare  form  which  is  put  down  as  congenital. 
In  this  variety  the  vision  of  the  defective  eye  is 
usually  extremely  defective,  so  that  the  patient  may 
only  be  able  to  distinguish  the  movement  of  the  hand 
before  the  face.  No  improvement  is  to  be  expected 
in  these  cases;    the   cause  of  the  amblyopia  is  un- 


212  THE    EEFRACTION    OF    THE    EYE 

known.  The  second  variety  of  amblyopia  is  very 
common  and  is  tlie  result  of  non-use ;  it  consists 
partly  in  an  awkwardness  and  difficulty  in  using  the 
eye^  but  more  especially  to  a  real  loss  of  function, 
consisting  of  a  weakness  of  the  fusion  faculty.  Some- 
times the  patient  says  he  is  unable  to  see  with  the 
eye  at  all,  and  yet  when  encouraged  to  read  with  it 
and  with  the  proper  optical  correction  before  the  eye 
-^  or  even  -f-g  may  be  read;  here  practice  is  essential, 
and  the  earlier  the  age  at  which  exercises  are  started 
the  greater  the  chance  of  improving  the  vision  and 
establishing  binocular  vision. 

In  amblyopia  it  will  often  be  found  that  the  vision 
is  best  on  the  temporal  part  of  the  field,  that  part 
which  is  most  used  in  peripheral  vision. 

In  high  degrees  of  hypermetropia,  when  no  amount 
of  accommodation  can  make  vision  distinct,  squint  is 
less  likely  to  occur.  It  is  usually,  therefore,  in  cases 
of  from  2  to  4  D,  that  convergent  strabismus  is 
most  frequently  met  with,  and  it  generally  makes  its 
appearance  about  the  fourth  or  fifth  year, — as  soon, 
in  fact,  as  the  child  begins  to  look  much  at  near 
objects. 

Anxious  parents  frequently  have  all  sorts  of  excel- 
lent reasons  to  which  they  attribute  the  defect ;  they 
say  that  the  child  has  been  imitating  its  playmate,  or 
that  the  nurse  did  something  which  caused  it  to 
squint,  by  making  the  child  look  too  much  or  too 
constantly  in  one  direction. 

Any  cause  which  by  rendering  the  image  in  one 
eye  less  distinct  than  that  in  the  other,  as  a  nebula 


CON\^ERGENT    STRABISMUS  213 

an  ulcer  of  the  cornea,  a  difference  in  the  refraction 
of  the  two  eyes,  or  even  wearing*  a  shade  for  a  few 
days  for  some  trivial  complaint,  may,  where  hyper- 
metropia  is  present,  combine  to  produce  strabismus 
by  weakening  the  power  of  fusion;  the  impulse  for 
binocular  vision  is  lessened,  and  the  eye  in  which  the 
fainter  image  is  formed  converges. 

It  is  thus  seen  that  convergent  strabismus  gra- 
dually destroys  binocular  vision.  In  cases  of  hyper- 
metropia,  where  binocular  vision  does  not  exist  owing 
to  great  difference  in  the  refraction  of  the  two  eyes, 
divergent  strabismus  may  occasionally  occur. 

The  intimate  connection  betAveen  accommodation 
and  convergence,  together  with  the  method  of  the 
production  of  strabismus,  will  be  more  easily  under- 
stood by  carrying  out  some  such  simple  experiments 
as  the  following.  We  will  assume  the  observer  to  be 
emmetropic  :  the  strongest  concave  glass  with  which 
he,  having  binocular  vision  and  being  at  a  distance 
of  six  metres,  can  still  read  f,  is  the  measure  of  the 
relative  accommodation.  The  absolute  accommodation  is 
measured  by  the  strongest  concave  glass  with  which 
each  eye  separately  can  read  -|.  In  my  own  case, 
with  —  4  D.  before  each  eye,  |-  can  be  seen  singly 
and  distinctly, —4'5  D.  renders  it  indistinct,  and  each 
increase  in  the  glass  increases  the  indistinctness,  but 
produces  no  diplopia.  Separately  each  eye  can  over- 
come —  7  D.  Armed  Avith  —  4  D.  before  each  eye, 
I  am  able  to  see  |-  distinctly,  using,  of  course,  4  D.  of 
my  accommodation;  if  a  coloured  glass  be  placed 
before  one  eye,  homonymous  diplopia  at  once  appears. 


214  THE    EEFEACTION    OF    THE    EYE 

proving  that    one  eye    has    deviated   inwards;    with 

—  3D.  and  the  coloured  glass^  squint  was  produced, 
but  with  no  weaker  concave  glass. 

On  repeating  the  experiment  in  an  individual 
with  '5  D.  of  myopic  astigmatism  in  the  right  eye, 
and  emmetropia  in  the  left,  —  2D.  before  each  eye 
was  the  strongest  glass  with  which  -J  could  be  seen 
clearly  and  singly,  —  2'5  D.  did  not  render  it  indis- 
tinct, but  produced  diplopia.  The  absolute  accom- 
modation   for    each    eye    amounted  to    6    D.      With 

—  2D.  before  each  eye,  the  coloured  glass  was  placed 
before  the  astigmatic  one,  and  diplopia  was  pro- 
duced. With  —  ID.  and  the  coloured  glass  the 
result  was  the  same  except  that  the  two  images  were 
nearer  together.  With  —  '6  D.  actual  diplopia  was 
not  produced. 

These  experiments  require  but  little  explanation. 
In  my  own  case,  when  using  4  D.  of  accommodation, 
there  is  a  tendency  also  to  use  a  corresponding- 
amount  of  convergence ;  I  am  conscious  of  this 
muscular  disturbance  by  the  effort  I  make,  and  by 
a  feeling  amounting  almost  to  giddiness,  produced 
when  first  looking  through  the  —  4  D.  The  instinc- 
tive desire  to  see  clearly  and  singly  is  so  great,  that 
the  external  recti  contract,  thereby  balancing  the 
increased  contraction  of  the  internal  recti.  Any 
increase  of  my  accommodation  above  4  D.  when 
looking  at  -J  causes  the  letters  to  become  indistinct, 
the  desire  to  maintain  binocular  vision  being  greater 
than  that  for  clear  images.  On  placing  the  coloured 
glass  before  one  eye,  we  diminish  the  retinal  impres- 


DIVERGENT    STRABISMUS  215 

sion  in  tliat  eye  ;  the  demand  for  binocular  vision  is 
lessened,  the  external  recti  cease  to  act,  and  as  a 
result  of  the  increased  action  of  the  internal  recti 
squint  occurs. 

In  the  second  experiment  the  retinal  impression 
in  one  eye,  even  with  so  slight  an  amount  of  astigma- 
tism, is  reduced  so  that  with  2  D.  of  accommodation 
the  desire  for  clear  images  is  greater  than  that  for 
binocular  vision,  and  diplopia,  the  symptom  of  squint, 
appears. 

A  certain  number  of  cases  of  convergent  strabismus 
get  well  about  the  age  of  seventeen ;  this  is  most 
likely  to  take  place  when  the  vision  in  both  eyes  is 
good,  though  it  sometimes  occurs  even  where  a  high 
degree  of  amblyopia  is  present  and  binocular  vision 
cannot  exist.  An  unsatisfactory  explanation  some- 
times given  is,  that  as  the  accommodation  diminishes, 
the  time  at  length  arrives  when  the  amount  of  accom- 
modation at  the  patient\s  disposal  is  not  sufficient  to 
produce  clear  images ;  he  therefore  relaxes  his  accom- 
modation, and  with  it  extreme  convergence. 

Divergent  Concomitant  Strabismus  exists  when  one 
e^^e  only  fixes  the  object  looked  at,  the  other  deviat- 
ing outwards  (Fig.  97) ;  it  is  usually  dependent  on 
myopia,  a  state  of  refraction  in  which  the  converg- 
ence has  to  be  used  in  excess  of  the  accommodation  if 
an  image  is  to  be  formed  on  the  macula  of  each  eye ; 
but  divergent  strabismus  may  occur  in  any  eye  in 
which  binocular  vision  does  not  exist,  as  in  some 
cases  of  high  hypermetropia  or  astigmatism;  or  it 
may  result  from  a  too  free  division  of  the  internal 


216  THE    REFEACTION    OF    THE    EYE 

rectus  muscle,  in  attempting  to  cure  a  case  of  con- 
vergent strabismus.  Divergent  strabismus  is  also 
occasionally  met  witli  in  emmetropia  and  liyperme- 
tropia,  and  is  then  due  to  congenital  insufficiency  of 
the  convergence. 

Fig.  97. 


Divergent  concomitant  strabismus  is  much  less 
common  than  the   convergent  variety. 

In  myopia  the  antero-posterior  diameter  of  the 
eyeball  is  elongated,  the  range  of  movement  is  dimin- 
ished, and  the  extreme  convergence  Avhich  is  neces- 
sary to  enable  the  patient  to  see  objects  within  his  far 
point  tires  out  the  internal  recti  muscles,  giving  rise 


DIVEEGENT    STKABISMUS  217 

to  muscular  asthenopia ;  to  relieve  this^  one  of  the  in- 
ternal recti  gives  way,  and  the  eye  deviates  outwards. 

Sometimes  the  deviation  only  takes  place  after  the 
patient  has  been  working  some  time  and  the  eyes  feel 
fatigued;  in  others  it  is  only  noticed  when  looking 
at  objects  beyond  their  far  point.  Soon,  however,  the 
squint  becomes  constant,  and  a  divergent  strabismus 
once  established  usually  increases. 

In  high  myopia  which  is  uncorrected  by  glasses, 
the  patient  has  to  hold  objects  so  close  to  enable  him 
to  see  them,  that  the  necessary  convergence  becomes 
impossible,  and  binocular  vision  is  therefore  sacrificed. 

In  the  treatment  of  concomitant  squint  our  objects 
are,  to  cure  the  deviation  of  the  eye,  and  to  retain  or 
re-establish  binocular  vision ;  the  subject  may  con- 
veniently be  divided  into  three  parts — 

1.  Optical. 

2.  Training  the  vision. 

3.  Operative. 

The  optical  treatment  consists  in  prescribing  glasses 
which  correct  any  error  of  refraction ;  by  doing  this 
we  prevent  excessive  accommodation,  and  equalise 
the  two  functions  of  accommodation  and  convergence. 

It  is  essential  that  in  all  patients  over  the  age  of 
one  year,  the  refractive  condition  of  the  eyes  should 
be  accurately  estimated  under  atropine  by  retino- 
scopy,  and  the  proper  correcting  glasses  ordered  for 
constant  use. 

In  a  new-born  child  squint  is  often  present,  the 
eyes  move  independently  of  each  other  and  without 
purpose,  the  vision  being  no   doubt   very  imperfect. 


218  THE    KEFEACTION    OF    THE    EYE 

but  gradually  as  the  brain-centres  develop,  the  two 
eyes  move  together  and  binocular  vision  becomes 
gradually  acquired_,  probably  between  the  sixth  and 
eighth  month,  so  that  squint  before  this  age  is  not  of 
much  importance ;  but  at  the  age  of  one  year  squint 
is  abnormal  and  should  receive  immediate  attention. 

In  order  that  the  squinting  eye  may  be  used  in 
young  children  and  not  become  amblyopic  it  is  an 
excellent  plan  to  put  one  drop  of  a  h  per  cent,  solution 
of  atropine  into  the  fixing  eye  twice  a  week ;  by  doing 
this  we  prevent  this  eye  being  used  for  near  objects, 
compelling  the  deviating  eye  to  be  employed  for  this 
purpose. 

In  some  cases,  when  the  squint  has  only  just  com- 
menced and  arises  only  under  the  influence  of  ex- 
cessive accommodation  necessary  to  enable  the  child 
to  see  near  objects  (periodic  squint),  this  treatment 
may  at  once  correct  the  deviation  of  the  eye,  and  the 
glasses  may  only  be  necessary  for  constant  use  for  a 
year  or  two,  but  must,  of  course,  be  continued  for 
near  work  for  a  much  longer  time. 

When,  however,  the  squint  has  already  become  per- 
manent, the  glasses  may  have  to  be  worn  constantly 
for  many  years.  Young  children  should  always  be 
encouraged  to  look  at  distant  objects  rather  than  near 
ones. 

It  will  frequently  be  noticed  that  after  the  use  of 
atropine  the  deviation  may  be  diminished,  or  in 
slight  cases  it  may  have  disappeared ;  this  is  due  to 
accommodation  being  rendered  impossible  :  these  are 
the  cases  that  are  usually  curable  by  glasses. 


STRABISMUS  219 

If  the  case  belong  to  the  less  common  variety  of 
squint — divergent  with  myopia — we  endeavour  to 
give  the  patient  as  near  as  possible  his  full  correction 
for  constant  use. 

2.  Training  the  vision. — In  most  cases  it  will  be 
found  that  even  when  the  deviation  of  the  eye  has 
been  corrected  binocular  vision  is  not  obtained,  and 
the  cure  of  the  case  cannot  be  considered  complete  as 
long  as  binocular  vision  is  absent;  great  patience  and 
care  will  be  required  in  carrying  out  the  necessary 
exercises. 

The  development  and  cultivation  of  the  fusion 
faculty  should  be  carried  out  by  the  use  of  one  of  the 
many  stereoscopes,  of  which  Worth^s  amblyoscope  is 
the  best ;  it  consists  of  two  halves  joined  together  by 
a  hinge;  each  half  consists  of  a  short  brass  tube 
joined  to  a  longer  tube  at  120°;  at  the  angle  of  junc- 
tion of  the  tubes  is  an  oval  mirror,  protected  on  the 
outside  by  an  oval  plate  of  brass.  Each  half  of  the 
instrument  has  at  its  distal  end  an  object  slide 
carrier,  and  at  its  proximal  end  a  convex  lens,  having 
a  focal  length  of  5  inches — the  distance  of  the  reflected 
image  of  the  object  slide ;  in  front  of  each  lens  is  a  slot 
into  which  a  prism  axis  vertical  may  be  inserted  if 
required;  the  diameter  of  the  tubes  is  1|  inch. 

A  brass  arc  connects  the  two  parts  of  the  instru- 
ment, being  clamped  on  one  side  by  a  binding  screw 
set  in  a  long  slot  and  on  the  other  by  a  binding  screw 
set  in  a  short  slot.  When  the  screw  of  the  long  slot 
is  loosened  the  two  parts  of  the  instrument  can  be 
brought  together  to  suit  a  convergence  of  the  visual 


220  THE    REFEACTION    OF    THE    EYE 

axis  up  to  60°  or  separated  to  suit  a  divergence  of  as 

much  as  30°. 

Fig.  98. 


When  this  screw  is  tightened  and  the  screw  in  the 
short  slot  loosened  an  amplitude  of  movement  of 
about  10°  only  is  permitted.  The  convex  lenses,  of 
course,  render  unnecessary  any  adjustment  of  the 
instrument  for  the  patient^s  interpupillary  width. 

Each  object  slide  is  illuminated  by  a  separate 
electric  light  so  arranged  that  the  illumination  of 
either  of  the  object  slides  may  be  separately  increased 
or  diminished  by  bringing  its  lamp  nearer  or  pushing 
it  further  away. 

The  object  slides  used  with  this  instrument  are  of 
three  kinds — 

(a)  Those  which  require  only  simultaneous  vision. 

(b)  Objects  which  require  true  fusion. 

(c)  Devices  which  can  only  be  appreciated  by 
persons  having  a  sense  of  perspective. 

For  a  full  description  of  the  use  of  the  amblyoscope 
I  must  refer  the  reader  to  Worth's  recent  work  on 
*  Squint.' 


STEABISMUS  22 1 

A  very  cheap  and  convenient  stereoscope  for  home 
use  has  been  introduced  by  my  colleague,  Mr. 
Brooksbank  James. 

Box  stereoscopes  are  also  useful.  They  are  made 
without  prisms,  but  fitted  with  a  clip  at  each  sight- 
hole  >3apable  of  taking  the  lenses  of  the  ordinary  trial 
box.  The  patient  being  emmetropic  he  will  require 
in  the  clip  a  convex  glass  whose  focal  length  cor- 
responds with  the  length  of  the  stereoscope  ;  thus  if 
it  be  16  cm.  long,  he  will  require  +  6  D. ;  should  the 
patient  be  hypermetropic,  3  D. ;  then  he  will  require 
+  9  D. ;  if  myopic  3  D.,  then  +  3  D.  would  be  the 
glass  required :  the  object  is  to  enable  him  to  see  the 
slide  at  the  end  of  the  stereoscope  without  accom- 
modating. 

A  convenient  slide  may  be  made,  composed  of  two 
vertical  lines,  one  above  and  the  other  below  the  same 
horizontal  line,  so  arranged  that  the  two  lines  can  be 
made  to  recede  or  approach  each  other :  this  test 
object  is  placed  in  the  box  instead  of  the  ordinary 
views.  The  two  lines  being  now  separated  to  a  dis- 
tance equal  to  that  between  the  two  eyes,  and  the 
clips  containing  the  necessary  convex  glasses,  the 
patient  will  see  the  lines  without  accommodation  or 
convergence,  and  should  succeed  in  fusing  the  two 
lines  into  one.  When  this  is  done  binocular  vision  is 
obtained  with  parallelism  of  the  lines  of  fixation.  We 
endeavour  at  future  trials  to  obtain  fusion  with  an 
equal  amount  of  convergence  and  accommodation. 
This  is  done  by  sliding  the  two  lines  towards  each 
other  about  1  cm. ;  this  will  call  into  play  something 


222 


THE    REFEACTION    OF    THE    EYE 


like  1  m.  a.  of  convergence;  we  then  diminisli  tlie 
convex  glass  1  T).,  so  that  the  amount  of  accommoda- 
tion provoked  (1  D.)  may  correspond  to  the  amount  of 


Fig.  99. 


convergence.  In  this  way  we  slide  the  lines  nearer 
and  nearer  together,  diminishing  the  +  glasses  at  the 
same  time,  until  the  two  form  one  vertical  line,  then 
binocular  vision  is  obtained  with  6  m.  a.  of  convergence 
and  with  6  D.  of  accommodation ;  when  this  point  has 
been  reached,  stereoscopic  pictures  may  be  used  as 
slides. 

When  the  deviating  eye  is  amblyopic  and  has  lost 
central  fixation,  the  fixing  eye  should  be  covered  up 
for  an  hour  twice  a  day  by  means  of  a  pad  or  shade 
or  a  small  black  metal  disc  may  be  made  to  slip  over 
the  spectacle  glass  of  this  eye,  great  care  being  taken 
that  the  eye  is  completely  covered  or  the  child  will 
try  to  dodge  the  shade. 

Another  most  useful  exercise  is  the  reading  bar, 
which  may  be  used  for  a  short  time  twice  a  day 
— a  pencil,  the  finger,  or  a  small  strip  of  metal  will 
answer  the  purpose ;  this  is  merely  to  be  held  about 
6  or  7  cm.  in  front  of  the  book  when  reading;  if  only 
one  eye  is  used  the  patient  stops  when  he  comes  to 
that  part  of  the  line  covered  by  the  bar ;  when  both 


STEABISMUS  223 

eyes  are  in  use^  then  lie  goes  on  reading  without  any 
interruption. 

Herring^s  drop  test  is  good  for  finding  out  the 
presence  of  binocular  vision  with  a  sense  of  perspec- 
tive, but  is  not  absolutely  reliable.  This  test  is 
seldom  used  clinically. 

Operative  treatment. — In  many  cases,  after  the 
glasses  have  been  worn  for  some  months,  the  stra- 
bismus may  still  exist,  and  it  may  then  be  necessary 
to  supplement  the  treatment  by  tenotomy.  A  free 
division  of  one  muscle  may  cure  a  deviation  of  15°  : 
when  a  greater  effect  is  required  an  advancement  of 
one  of  the  muscles  may  be  necessary,  with  or  without 
a  tenotomy  of  its  antagonist. 

After  the  operation  for  convergent  strabismus  there 
should  still  remain  a  slight  tendency  to  convergence 
when  the  glasses  are  removed ;  because  as  the  child 
approaches  the  age  of  maturity  the  excessive  in- 
nervation of  the  internal  recti  may  subside,  and  then 
there  may  be  danger  of  a  deviation  of  one  eye  out- 
wards. 

Paralytic  strabismus  does  not  come  within  the 
province  of  this  work. 

See  Case  35,  page  257. 


224  THE    REFRACTION    OP    THE    EYE 


CHAPTER  XI 


ASTHENOPIA 


Asthenopia  (^A,  priv. ;  aOtvo^,  strength ;  io\p,  the 
eye),  or  weak  sight,  is  a  term  used  to  designate  a 
group  of  symptoms  caused  by  fatigue  or  strain  of 
some  part  of  the  eye  or  its  muscles. 

Asthenopia  frequently  accompanies  hypermetropia, 
myopia,  and  astigmatism,  and  reference  has  often 
been  made  to  it  when  speaking  of  these  errors  of 
refraction.  It  is  also  met  with  in  cases  where  no 
ametropia  exists,  and  may  then  be  caused  by  over- 
fatigue or  diminished  power  of  the  ciliary  muscles, 
weakness  of  the  internal  recti,  or  exhaustion  of  an 
over-sensitive  retina. 

Asthenopia  shows  itself  by  the  inability  to  sustain 
long  and  continuous  near  work,  and  is  accompanied 
w^ith  more  or  less  pain ;  the  condition  is  a  very  common 
one,  and  it  may  be  stated  with  confidence  that  pain 
in  the  eyes,  unconnected  with  inflammation,  is  almost 
invariably  due  to  asthenopia,  and  but  seldom  to  any 
deep-seated  disease.  The  more  acute  the  pain,  the 
more  does  it  point  to  asthenopia ;  as  a  rule,  however, 
the  pain  is  not  very  severe :  it  may  be  situated  in  the 
eyes  themselves,  or  around  the  orbits,  and  is  always 


ASTHENOPIA  225 

increased  when  the  eyes  are  used  for  near  objects ;  in 
some  cases  no  pain  is  felt^  but  after  reading  for  a 
while  the  type  becomes  indistinct  or  double,  so  that 
the  patient  has  to  stop  and  look  about  the  room,  or 
rub  his  eyes,  after  which  he  may  be  able  to  resume 
reading  for  a  short  time,  to  be  again  quickly  inter- 
rupted by  a  repetition  of  the  same  symptoms.  If  the 
work  be  still  persisted  in,  the  pain  around  the  eyes 
increases,  there  is  photophobia  and  lachrymation,  a 
sensation  of  dazzling  and  dimness,  more  or  less  con- 
junctival congestion,  the  eyes  and  lids  becoming  red 
and  irritable;  all  these  symptoms  are  liable  to  be 
worse  in  the  evening  after  a  day^s  work,  when  there 
is  the  additional  disadvantage  of  an  artificial  light, 
which  may  be  hot  and  irritating. 

Headache  is  often  a  prominent  symptom  of  asthe- 
nopia j  it  may  take  the  form  of  heaviness  or  pain 
over  the  brow  (which  may  or  may  not  be  combined 
with  general  headache)  ;  it  is  often  periodic  in 
character,  and  is  always  made  worse  by  reading ; 
frequently  there  is  a  tender  spot  on  the  top  of  the 
head,  or  pain  in  the  occipital  region,  occasionally  also 
there  is  pain  in  the  back  of  the  neck.  These  symptoms 
may  be  associated  with  dyspepsia,  palpitation,  and 
vomiting,  and  in  some  cases  with  obstinate  insomnia. 

This  train  of  symptoms  has  occasionally  been  so 
severe  as  to  lead  to  the  diagnosis  of  brain  disease, 
hence  it  is  a  good  rule  to  test  the  refraction  under 
atropine  in  all  cases  of  persistent  headache  not  giving 
way  to  ordinary  medical  treatment,  and  it  must  be 
remembered  that  a  very  slight  amount  of  astigmatism 

15 


226  THE    EEFEACTION    OP    THE    EYE 

left  uncorrected^  even  though  the  chief  portion  of  it 
may  be  corrected,  will  be  sufficient  in  some  cases  to 
keep  up  the  headache. 

There  is  little  doubt  that  frequently  reflex  nervous 
disorders  are  caused  by  asthenopia. 

Asthenopia  may  be  divided  into — 

1.  Accommodative. 

2.  Muscular. 

3.  Retinal. 

Accommodative  Asthenopia  is  exceedingly  common, 
and  is  due  to  fatigue  of  the  ciliary  muscle ;  and  may 
be  met  with  in  emmetropia  or  ametropia.  It  makes 
itself  known  by  an  inability  to  maintain  the  necessary 
accommodation,  and  may  arise  (a)  from  a  weak  con- 
dition of  the  ciliary  muscle,  (h)  from  excessive  use, 
as  in  hypermetropia,  (c)  from  unequal  demand,  as  in 
astigmatism,  [d)  from  unequal  demand  in  the  two 
eyes,  as  in  anisometropia,  (e)  from  diminished  elasticity 
of  the  lens,  as  in  presbyopia. 

When  Bonders  discovered  the  common  occurrence 
of  hypermetropia,  he  soon  became  aware  of  the  very 
intimate  connection  which  existed  between  it  and 
asthenopia,  and  was  at  first  inclined  to  attribute  every 
case  to  this  cause.  Where  no  manifest  hyperme- 
tropia was  present  he  gave  a  solution  of  atropine  to 
paralyse  the  accommodation,  feeling  confident  that 
some  latent  hypermetropia  would  then  display  itself ; 
such  cases  were  usually  completely  cured  by  proper 
convex  glasses.  Accommodative  asthenopia  is  due 
in  great  measure  to  the  constant  and  excessive 
action  of  the  ciliary  muscle,  but  partly  also  to   the 


ACCOMMODATIVE    ASTHENOPIA  227 

abnormal  relations  existing  between  tlie  two  func- 
tions^ accommodation  and  convergence;  this  state- 
ment is  supported  by  the  fact  that  hypermetropes 
who  squint  seldom  suffer  from  asthenopia. 

An  emmetrope  looking  at  distant  objects  does  so 
without  any  accommodation,  the  ciliary  muscle  being 
passive  ;  but  the  hypermetrope  has  to  use  his  ciliary 
muscle  even  for  distant  objects,  and  therefore  much 
more  for  reading  or  near  work ;  so  that  the  ciliary 
muscle  practically  gets  no  rest.  A  young  and  vigorous 
person  whose  hypermetropia  is  not  very  high  may 
resist  asthenopia  for  a  long  time,  but  as  he  gets  older, 
or  if  his  health  suffer  from  any  cause,  symptoms 
of  this  disorder  are  apt  to  appear,  and  when  once 
established  they  may  continue,  notwithstanding  im- 
provement in  the  patient's  general  condition.  In 
women  asthenopia  is  very  liable  to  come  on  during 
lactation. 

Treatment. — We  order  such  glasses  as  are  neces- 
sary to  correct  the  refraction  according  to  the  rules 
given.  In  some  cases  where  convex  glasses  do  not 
produce  the  desired  relief,  prisms  of  2°  bases  iuAvards 
combined  with  the  spherical  correction  are  of  great 
use,  or  in  slight  cases  we  place  the  convex  glasses 
somewhat  near  together,  so  that  the  patient  looks 
through  the  outer  part  of  them  (Fig.  102).  This  plan 
is  frequently  very  useful  in  presbyopia.  Here  the 
asthenopia  is  due  to  a  greater  muscular  effort  being 
required  to  produce  the  necessary  change  in  the 
shape  of  the  less  elastic  lens,  and  perhaps,  also,  in 
part  to  the  difficulty  of  maintaining  an  exact  state  of 


228  THE    REFRACTION    OF    THE    EYE 

equilibrium  between  tlie  internal  and  external  recti 
muscles. 

In  the  hypermetrope  there  is  a  want  of  harmony 
between  the  accommodation  and  the  convergence^ 
the  two  functions  having  to  be  used  in  unequal 
degrees;  and  when  we  correct  his  refraction  with 
glasses  he  will  have  to  use  these  two  functions 
equally,  or  at  least  in  different  proportions  from  that 
to  which  he  has  been  accustomed.  Many  people  are 
able  at  once  to  adapt  themselves  to  this  new  state  of 
aifairs ;  but  there  are  others  in  whom  the  force  of 
habit  is  so  strong  that  they  cannot  thus  throw  off  the 
acquired  one  of  using  the  accommodation  in  excess  of 
the  convergence.  You  must  not,  therefore,  be  dis- 
couraged if  occasionally  your  patient  is  not  at  once 
and  completely  relieved  of  his  asthenopia,  as  soon 
as  you  have  given  him  convex  spectacles.  A  fort- 
night's trial  should  be  made  before  we  can  decide 
that  sugh  spectacles  will  not  relieve  the  patient  of  his 
asthenopia. 

Asthenopia  depends  much  upon  the  nervous  system 
of  the  individual;  in  some,  a  very  slight  amount 
of  astigmatism  will  produce  accommodative  asthe- 
nopia ;  one  hypermetrope  will  have  no  uncomfortable 
feelings,  while  another,  whose  condition  seems  exactly 
similar,  will  suffer  much,  so  that  it  is  essential  to 
attend  to  the  patient's  general  health. 

Muscular  Asthenopia  is  due  to  fatigue  of  some  of 
the  external  muscles  of  the  eyeballs. 

The  two  eyes  should  be  in  a  condition  of  perfect 
muscular  equilibrium  in  every  position  of  the  eyes; 


MUSCULAR   ASTHENOPIA  229 

wlien  this  is  not  the  case,  it  may  be  detected  by 
covering  one  eye  witli  a  screen,  while  the  other  is 
made  to  fix  a  small  object,  such  as  the  point  of  a  pen, 
held  about  30  cm.  away ;  the  covered  eye  should 
be  accurately  adjusted  for  the  object,  although  it  no 
longer  sees  it :  when  this  is  not  the  case  the  covered 
eye  may  deviate  inwards  or  outwards ;  and  if  the 
screen  now  be  withdrawn  a  movement  of  readjust- 
ment takes  place. 

Maddox  has  pointed  out  that  in  practice  it  will 
be  found  that  on  using  the  two  functions  of  accom- 
modation and  convergence,  the  convergence  has  a 
tendency  to  lag  behind  the  accommodation  and  re- 
quires the  further  stimulus  of  fusion  to  ensure  the 
exact  direction  of  the  visual  axes,  so  that  Avhen  one 
eye  is  covered  the  other  may  deviate  outwards  a  few 
degrees  and  still  be  considered  within  normal  limits. 

When  the  deviation  of  the  covered  eye  is  greater 
than  5°,  then  there  is  disturbance  of  the  muscular 
equilibrium,  and  the  condition  is  spoken  of  as  in- 
sufficiency or  latent  convergence  or  divergence. 

When  insufficiency  exists  the  results  which  follow 
vary. 

1.  Slight  degrees  may  be  corrected  by  increased 
innervation  of  the  weak  muscles,  and  so  give  rise  to 
no  symptoms. 

2.  Muscular  asthenopia  may  result  from  excessive 
innervation ;  this  may  at  once  be  relieved  by  closing 
one  eye. 

3.  Or  concomitant  squint  may  develop. 
Heterophoria  is  the  term  now  generally  employed 


230  THE    REFEACTION    OF    THE    EYE 

to   express   a   disturbance   of  the  equilibrium  of  the 
muscles  of  the  eyeball,  and  may  be  divided  into — 

1.  Exophoria. 

2.  Esophoria. 

3.  Hyperphoria. 

4.  Insufficiency  of  the  oblique  muscles. 

a.  Hyperesophoria. 
j3.  Hyperexophoria. 

Exoj)horia,  or  latent  divergence ;  one  eye  tends  to 
turn  out,  and  is  only  prevented  from  doing  so  by 
increased  innervation  of  the  internal  recti  muscles, 
there  is,  in  fact,  insufficiency  of  these  muscles,  resulting 
in  a  strain  of  the  convergence. 

Exophoria  is  the  commonest  of  these  defects,  and 
is  most  frequently  associated  with  myopia,  though  it 
occasionally  occurs  in  emmetropia  or  even  hyperme- 
tropia ;  it  is  characterised  by  inability  to  maintain 
prolonged  convergence.  The  patient  complains  that 
the  eyes  become  tired,  especially  during  the  evenings, 
reading  or  writing  cannot  be  continued  for  any 
length  of  time ;  he  has  pain  in  and  around  the 
eyes,  with  headache ;  objects  look  dim  and  indis- 
tinct, and  there  is  a  tendency  to  see  things  double  ; 
sometimes  the  patient  experiences  a  sensation  as  if 
one  eye  had  turned  outwards,  which  may  really  be 
the  case  ;  frequently  the  patient  finds  relief  by  closing- 
one  eye. 

In  myopia  the  disturbance  of  the  two  functions 
accommodation  and  convergence,  may  in  some  mea- 
sure tend  to  the  production  of  this  form  of  asthenopia. 
Thus  a  patient  with  4  D.  of  myopia  has  his  punctum 


HETEEOPHOEIA  231 

remotum  at  25  cm. ;  to  see  an  object  at  that  distance 
he  must  converge  to  that  pointy  maintaining  at  the 
same  time  a  passive  condition  of  his  accommodation. 

Strain  of  the  internal  recti  muscles  is  essentially 
dependent  upon  binocular  vision;  no  convergent  strain 
can  exist  where  binocular  vision  is  not  present. 

Esophoria,  or  latent  convergence^  is  due  to  insuffi- 
ciency of  the  external  recti,  and  is  seldom  seen,  for  it 
quickly  passes  on  to  convergent  squint  with  loss  of 
binocular  vision ;  here,  as  a  rule,  we  are  dealing  with 
excessive  innervation  of  the  internal  recti  muscles 
rather  than  with  an  insufficiency  of  their  opponents. 

Hyperphoria,  one  eye  tends  to  deviate  upwards ; 
this  is  usually  associated  with  esophoria,  but  may 
exist  alone  ;  this  is  not  a  very  uncommon  defect. 

Insufficiency  of  the  obliques  I  have  never  seen,  and 
no  cases  have  been  recorded  in  this  country,  though 
they  seem  to  occur  in  America. 

To  test  and  record  the  amount  of  latent  deviation, 
the  glass-rod  test  described  on  page  46  may  be  em- 
ployed. The  patient  is  directed  to  look  at  the  flame 
of  a  candle  6  metres  away ;  immediately  behind  the 
flame  is  a  scale  for  measuring  the  amount  of  devia- 
tion ;  the  glass  rod  is  then  placed  horizontally  before 
the  right  eye-  in  testing  for  exophoria  or  esophoria ; 
if  the  streak  of  light  appear  on  the  right  of  the 
candle  homonymous  diplopia  is  present,  and  the  con- 
dition of  the  eyes  is  one  of  convergence  :  whereas  if 
the  streak  has  its  position  on  the  left  side  of  the 
candle,  crossed  diplopia  is  present,  and  the  eyes  are 
divergent ;  the   number   on  the   scale   corresponding 


232 


THE    EEFEACTION    OF    THE    EYE 


Fig.  100. 


witli  tlie  position  of  tlie  streak  of  light  indicates  the 
amount  of  convergence  or  divergence.  When  em- 
ployed for  hyperphoria  the  rod  must  be  placed  before 
the  eye  vertically. 

Another  test  for  detecting  insufficiency  of  the  con- 
vergence is  sometimes  employed. 

Place  a  prism  of  about  12°,  with  its  base  down- 
wards, in  a  spectacle  frame  before  one 
eye  :  by  this  means  we  cause  a  displace- 
ment of  the  eye  upwards  which  produces 
vertical  diplopia.  The  patient  is  now 
directed  to  look  steadily  at  a  card,  on 
which  is  drawn  a  line  with  a  dot  in  its 
centre,  placed  at  the  patient's  ordinary 
reading  distance  (Fig.  TOO).  Naturally  he 
will  see  two  dots.  If  he  see  one  line  only 
with  two  dots  on  it,  his  muscles  are 
assumed  to  be  of  the  normal  strength ; 
if,  however,  two  lines  are  seen  with  a  dot 
on  each,  then  insufficiency  exists,  and 
the  strength  of  the  prism  which  is  neces- 
sary with  its  base  inwards  to  produce  fusion 
is  the  measure  of  the  insufficiency. 

The  most  satisfactory  test  for  muscular 
insufficiency  is,  however,  the  rod  test ;  and 
having  recorded  the  amount  of  lateMt  con- 
vergence or  divergence  for  distance,  we  next  ascertain 
if  there  is  any  latent  deviation  in  near  vision. 

A  prism  of  12°  base  upwards  being  placed  before 
the  right  eye  in  a  spectacle  frame,  the  scale  is  held 
\  metre  from  the  eyes.     The  scale  used  by  Maddox 


CO  — 


OJ 


< 


CM 


CO 


MUSCULAR    ASTHENOPIA  233 

consists  of  a  horizontal  line  in  tlie  centre  of  which  is 
an  arrow  pointing  upwards  (Fig.  101).  The  line  is 
divided  into  metre  angles  which  are  marked  by  figures, 
black  on  the  right  of  the  arrow,  red  on  the  left.  The 
prism  of  course  causes  two  arrows  and  two  lines  to  be 
seen :  the  patient  is  directed  to  fix  the  fine  print  just 
below  the  arrow,  and  if  there  is  no  deviation  the 
two  arrows  will  be  seen,  one  immediately  below  the 
other;  if  the  lower  one  point  to  the  right  (black 
figures)  there  is  latent  convergence,  but  if  to  the 
left  (red  figures)  latent  divergence  for  this  distance 
the  amount  of  deviation  is  read  off  the  scale  and  duly 
recorded. 

Treatment. — In  cases  of  myopia  we  give  such 
glasses  as  correct  the  refraction  to  be  worn  con- 
stantly ;  these  frequently  succeed  in  relieving  the 
asthenopia.  When  this  is  not  the  case,  weak  prisms 
bases  inwards,  by  which  we  diminish  the  amount  of 
convergence  necessary,  often  give  instant  relief.  It 
is  sometimes  useful  to  combine  the  prisms  with 
concave  glasses,  or  by  separating  the  glasses  some- 
what widely  we  may  produce  the  same  result.  Fig. 
102  shows  concave  spectacles,  which  act  as  prisms 
by  being  slightly  separated;  convex  glasses  have 
the  same  effect  when  placed  so  close  together  that  the 
patient  looks  through  the  outer  part  of  the  lenses. 
Or  the  lenses,  instead  of  being  thus  displaced,  may  be 
decentred  by  so  grinding  the  glass  that  its  optical 
centre  is  not  the  centre  of  the  glass.  These  de- 
centred  glasses  are  spoken  of  as  frismosjpheres,  and 
when  ordered  the  amount  of  decentration  should  be 


234  THE    EEFRACTION    OF    THE    EYE 

stated  in  millimetres  on  the  order  card ;  the  more  the 
glasses  are  decentred  the  greater  will  be  the  pris- 
matic effect  produced  (page  194). 

When  actual  divergence  of  one  eye  takes  place, 
advancement  of  one  internal  rectus,  with  or  without 
division  of  its  antagonist,  may  be  necessary. 


Retinal  Asthenopia  is  due  to  fatigue  of  the  retina. 

In  addition  to  those  cases  of  asthenopia  occurring 
with  hypermetropia,  myopia,  and  astigmatism,  which 
should  be  relieved  by  the  proper   optical  correction 


EETINAL    ASTHENOPIA  235 

restoring  the  balance  of  the  extra-  and  intra-ocular 
muscular  systems,  every  one  will  occasionally  meet 
with  cases  where  there  is  intense  discomfort  and 
inability  to  read  or  do  near  work  for  any  length  of 
time,  but  where  no  ametropia  exists,  as  proved  by 
placing  the  patient  under  atropine  and  then  testing 
the  refraction.  The  visual  acuteness  is  often  very 
good,  frequently  rising  to  |-  or  higher.  The  pain 
complained  of  is  usually  at  the  back  of  the  eyes,  with 
more  or  less  headache,  photophobia,  lachrymation,  a 
feeling  of  tension  and  heat,  together  with  itching 
and  pricking  of  the  eyelids.  Sometimes  the  chief 
symptom  complained  of  is  the  conjunctival  irritation 
accompanied  with  increased  secretion. 

These  cases  are  always  exceedingly  troublesome 
and  difficult  to  cure;  they  occur  most  frequently 
among  young  unmarried  girls  of  an  hysterical  or 
nervous  temperament.  Less  frequently  men  are 
affected,  and  then  it  is  chiefly  amongst  those  who  are 
feeble,  hypochondriacal,  and  nervous. 

With  the  ophthalmoscope  the  eyes  may  appear 
quite  normal,  or  the  retinal  veins  may  be  full  with  or 
without  some  slight  haziness  of  the  edges  of  the  discs; 
the  perimeter  may  reveal  a  spiral  contraction  of  the 
visual  fields  due  to  progressive  exhaustion  of  the 
retina,  which  probably  follows  reflex  contraction  of 
the  retinal  vessels. 

Retinal  asthenopia  may  be  attributed  to  long  hours 
of  near  work  which  has  been  done  by  artificial  light, 
especially  in  those  who  have  been  previously  reduced 
by  some  lowering  illness.     I   have  met  with   several 


236  THE    REFRACTION    OF    THE    EYE 

cases  amongst  those  making  gold  lace^  and  no  doubt 
the  briofht  materials  here  worked  with  had  somethins: 
to  do  with  the  production  of  the  retinal  hyper^es- 
thesia. 

It  seems  generally  accepted  by  all  authorities  on 
this  subject  that  in  most  cases  the  nervous  system  is 
exceedingly  sensitive  and  unstable. 

Sometimes  the  asthenopia  is  distinctly  of  reflex 
origin^  produced  by  disturbance  of  the  internal  organs; 
when  leucorrhoea  exists  in  young  unmarried  women, 
with  troublesome  asthenopia,  masturbation  may  be 
suspected.  Irritation  of  the  fifth  nerve  from  carious 
teeth  may  also  act  as  the  exciting  cause. 

Treatment. — Complete  abstinence  from  near  work 
does  not  always  give  relief,  nor  is  this  abstinence  to  be 
encouraged.  A  slight  amount  of  regular  work  should 
be  done  every  day,  with  rest  for  the  eyes  during  the 
evening.  Usually  some  form  of  nerve  tonic  is  indicated, 
with  plenty  of  outdoor  exercise  and  the  avoidance  of 
strong  light  or  places  where  there  is  a  great  glare, 
such  as  the  sea-side  in  summer.  Tinted  glasses  are 
to  be  avoided  under  ordinary  conditions  of  light, 
since  they  merely  tend  to  increase  the  hyper^esthesia 
of  the  retina. 


SPECTACLES  237 


CHAPTER  XII 

SPECTACLES 

Having  referred  to  the  subject  of  spectacles  when 
considering  the  correction  of  the  different  forms  of 
ametropia^  I  will  noAV  briefly  recapitulate  what  was 
then  said^  even  at  the  risk  of  being  accused  of  un- 
necessary repetition. 

Hypermetropia. — So  long  as  -J  can  be  read  with 
each  eye^  no  glass  is  necessary  for  distant  vision ;  for 
reading  and  near  work  we  give  such  glasses  as 
correct  the  manifest  and  4-  of  the  latent  hyperme- 
tropia. 

If  distant  vision  be  improved  by  convex  glasses^ 
then  these  may  be  prescribed. 

In  hypermetropia  complicated  with  strabismus  Ave 
estimate  the  total  hypermetropia  under  atropine^  then 
give  the  full  correction  to  be  worn  constantly. 

Myopia. — In  cases  of  low  degree  in  people  over 
twenty  years  of  age  we  may  prescribe  folders  for 
distance,  allowing  the  patient  to  read  and  write 
without  glasses  if  only  he  keep  a  sufficient  distance 
(30  cm.)  from  his  book  and  suffers  no  inconvenience ; 
under  the  age  of  twenty  a  full  correction  should  be 
worn  constantly.  In  medium  degrees  the  best  results 
often  ensue  when  the  full  correction  is  always  worn. 

Where  the  myopia  is  of  high  degree  the  full  cor- 


238  THE    EEFRACTION    OF    THE    EYE 

rection  may  be  satisfactory  for  distance^  but  uncom- 
fortable or  impossible  for  reading,  owing  to  the 
accommodation  being  insufficient.  These  glasses  also 
have  the  disadvantage  of  diminishing  the  size  of 
objects ;  here  we  give  two  pairs  of  spectacles,  one  for 
distance,  and  a  weaker  pair  for  reading. 

Astigmatism. — Our  object  is  to  give  as  near  as 
possible  the  full  correction  (found  by  atropizing  the 
patient) ;  these  glasses  should  be  worn  constantly. 

Atropine  is  seldom  necessary  in  patients  over  twenty 
years  of  age.  Homatropine  and  cocaine  is  usually 
sufficient  in  older  people. 

Convex  glasses  render  parallel  rays  which  pass 
through  them  convergent :  they  add  therefore  to  the 
refraction  of  the  dioptric  system,  and  are  called 
positive. 

Concave  glasses  render  parallel  rays  divergent ; 
they  therefore  diminish  the  refraction  of  the  dioptric 
system,  and  are  called  negative. 

Convex  glasses  add  to  the  quantity  of  light  entering 
the  eye,  while  concave  glasses  diminish  it. 

The  size  of  the  image  is  modified :  thus  positive 
glasses  bring  forward  the  nodal  point,  and  so  increase 
the  size  of  the  image  ;  while  negative  glasses  carry 
the  nodal  point  backwards,  and  so  diminish  the  size  of 
the  image. 

Glasses  may  be  made  of  rock  crystal  (commonly 
called  pebbles)  or  crown  glass.  Those  made  from  the 
former  material  have  the  advantage  of  being  harder, 
and  are  therefore  less  likely  to  be  scratched  than 
glass;   the  weight  is  much  the  same  in   both  cases. 


SPECTACLES  239 

Pebbles  absorb  more  heat,  and  unless  cut  exactly  at 
right  angles  to  their  optic  axis  tliey  are  apt  to  refract 
unequally;  besides,  it  is  difficult  to  get  rock  crystal 
free  from  strise,  so  that  lenses  made  from  good  crown 
glass  are  quite  equal  to  the  best  pebbles,  very  much 
cheaper,  and  almost  universally  used. 

The  method  of  mounting  spectacle  glasses  is  of  the 
greatest  importance ;  they  must  be  accurately  centred 
in  frames  that  are  light,  strong,  and  fit  well,  other- 
wise the  good  effect  of  the  most  carefully  chosen 
correction  may  be  entirely  frustrated  by  a  faulty 
position  of  the  glasses,  or  even  a  fresh  source  of  eye- 
strain may  be  introduced.  Gllasses  for  constant  use 
should  be  in  the  same  plane  and  centred  for  distance; 
those  intended  for  near  work  only  should  converge 
slightly,  be  centred  for  the  reading  distance,  and  be 
inclined  downwards  15°  to  20°. 

Each  lens  should  be  in  the  first  focal  plane  of  the 
eye,  that  is  13*7  mm.  in  front  of  the  cornea.  When 
this  is  the  case,  the  images  formed  on  the  retina  will 
be  of  the  same  size  as  in  emmetropia. 

The  bridge  of  the  frame  should  be  moulded  to  suit 
the  shape  of  the  nose,  resting  on  it  by  a  broadish 
surface  so  as  not  to  cut  or  indent  the  skin,  while  the 
glasses  should  be  a  sufficient  distance  from  the  eyes  to 
just  clear  the  lashes.  The  sides  of  the  frames  should 
pass  back  immediately  above  the  ears,  and  in  many 
cases,  especially  where  glasses  are  required  to  be  worn 
constantly,  they  may  with  advantage  bend  directly 
round  the  ears,  fitting  the  posterior  part  of  the 
concha;  these    ear-pieces    may    be    made   of   twisted 


240  THE    EEFEACTION    OF    THE    EYE 

wire^  wliicli  gives  them  considerable  elasticity  and 
strength.  The  frames  may  be  made  of  gold,  rolled 
gold,  or  steel ;  the  latter  material  has  the  great  dis- 
advantage of  rusting  easily. 

When  glasses  are  worn  for  myopia  or  hyper- 
metropia  they  should  not  be  further  from  the  eye 
than  13*7  mm.  For  presbyopia  the  person  may  be 
allowed  to  suit  his  own  convenience  and  comfort, 
2h  cm.  being  an  ordinary  distance. 

Single  glasses  may  occasionally  be  allowed  in  low 
degrees  of  myopia  for  looking  at  distant  objects. 
They  have  the  disadvantage  of  encouraging  mon- 
ocular vision,  and  sometimes  one  eye  is  used  so 
entirely  that  the  sight  in  the  other  may  deteriorate 
from  want  of  sufficient  use. 

Folders  (pince-nez),  of  which  there  are  many  varie- 
ties, may  be  used  in  some  cases  of  hypermetropia 
and  myopia;  many  presbyopes  find  them  very  con- 
venient for  reading.  The  frameless  pince-nez  are 
very  neat  and  comfortable. 

Spectacles  are  as  a  rule  to  be  preferred  in  children, 
since  they  are  more  accurately  centred  and  fit  better. 

In  cases  of  astigmatism  it  was  formerly  the  custom 
to  order  spectacles  and  not  folders,  as  in  the  latter  it 
is  difficult  to  be  certain  that  the  cylindrical  glasses 
are  always  in  their  proper  axis ;  but  several  ingenious 
pince-nez  have  been  brought  out  which  are  free 
from  this  objection.  That  form  in  which  the  glasses 
slide  on  a  horizontal  bar  is  so  arranged  that  they  fit 
on  the  nose  very  easily,  and  are  extremely  comfort- 
able, they  may  be  recommended  in  many  cases. 


SPECTACLES  241 

In  addition  to  concave,  convex,  and  cylindrical 
glasses,  others  are  sometimes  used. 

Periscopic  glasses  are  an  advantage  in  many  cases. 
They  give  a  large  field,  and  there  is  less  spherical 
aberration. 

Bifocal  glasses  are  often  convenient  for  presbyopes 

Fig.  103. 


who  are  also  ametropic.  The  upper  part  of  the  glass 
is  then  used  for  distance,  the  lower  part  for  reading. 
There  are  several  varieties — 


Fig.  104. 


(a)  Made  in  two  halves  separately  (Fig.  103). 

(b)  Ground  on  one  glass  by  a  special  tool  (Fig.  104). 

(c)  The  invisible  bifocal,  which  consists  of  two  thin 
layers  of  glass,  between  which  is  placed  at  its  lower 

16 


242  THE    EEPRACTION    OF    THE    EYE 

part   a  thin   circular  lens  having  a  high  refractive 
index  (Fig.  105). 

Fig.  105. 


Stenojpaic  spectacles  consist  of  an  opaque  screen 
with  a  small  central  aperture  which  may  be  of  any 
shape  to  suit  a  particular  case^  so  that  all  the  peri- 
pheral rays  are  cut  oif^  only  those  that  are  in  the 
visual  axis  being  allowed  to  pass  through.  They  can 
be  combined  with  convex  or  concave  glasses^  and  are 
sometimes  exceedingly  useful  in  cases  of  leucomata, 
nebulae,  irregular  astigmatism,  conical  cornea,  etc., 
where  the  vision  is  much  disturbed. 

Prismatic  spectacles  may  consist  of  prisms  alone,  or 
they  may  be  in  combination  with  concave  or  convex 
lenses.  It  is  not  convenient  to  use  prisms  much 
stronger  than  3°  or  4°,  owing  to  their  weight  and  the 
chromatic  aberration  they  produce.  They  are  use- 
ful in  certain  cases  of  paralysis  of  muscles,  to  correct 
the  diplopia,  and  in  some  cases  of  hypermetropia, 
myopia,  and  astigmatism  which  are  not  relieved  by 
their  proper  correction ;  prisms  are  also  used  for 
testing  the  ocular  muscles  and  for  detecting  malin- 
gerers.    When  ordered  in  cases   of  asthenopia  with 


SPECTACLES  243 

errors  of  refraction,  they  may  be  combined  with  the 
glasses  which  correct  these  errors  (p.  194). 

Tinted  glasses  are  often  required  for  diminishing 
excessive  light,  especially  where  there  is  irritation 
or  inflammation  of  the  retina;  they  are  also  useful 
in  some  cases  of  photophobia,  arising  from  various 
causes,  as  myopia,  etc.  Where  the  aim  is  to  relieve 
the  retina  without  injuring  the  distinctness  of  vision 
the  light  blue  glasses  are  the  best,  as  they  cut  off 
the  orange  rays;  where  the  object  is  to  act  on  the 
quantity  and  not  the  quality  of  the  light,  smoke- 
coloured  glasses  are  to  be  preferred.  Tinted  glasses 
sometimes  do  real  harm,  as  in  cases  of  asthenopia,  by 
increasing  the  sensitiveness  of  the  retina;  they  are 
always  somewhat  heating  to  the  eyes,  in  proportion 
to  the  amount  of  rays  they  absorb.  We  sometimes 
combine  them  with  convex  or  concave  glasses. 

There  are  also  various  forms  of  protector^s ;  those 
hollowed  out  like  a  watch-glass,  so  as  to  fit  closely, 
are  to  be  preferred  to  those  with  wire  sides  called 
goggles,  or  those  with  sides  of  glass,  which  have  the 
disadvantage  of  being  heavy.  Workmen  wear  diife- 
rent  sorts  of  protectors  to  keep  off  dust,  fragments  of 
stone,  etc.,  which  may  be  made  of  glass,  talc,  wire 
gauze,  or  other  material. 

Shot-proof  spectacles  are  made  for  sportsmen. 

It  is  sometimes  necessary  to  find  out  and  record 
the  strength  of  glasses  that  are  being  worn ;  this  is 
easily  done.  If  convex,  we  take  a  concave  glass  out 
of  the  trial  box,  hold  it  against  the  glass  we  are 
trying,  and  look  through  them  at  a  line,  e.  g.  the  bar  of 


244  THE    EEFEACTION    OF    THE    EYE 

a  window  or  any  similar  object.  We  move  the  glasses 
to  and  fro  in  front  of  the  eye ;  if  the  line  remains 
immovable  the  neutralisation  is  complete ;  if  it  move 
in  the  same  direction  the  concave  glass  is  too  strong ; 
if  in  the  opposite  direction  it  is  not  strong  enough. 

The  Greneva  lens  measure  is  a  simple  instrument 
for  estimating  the  strength  of  lenses. 

Cases 

Commence  the  examination  in  a  systematic  manner. 

First,  notice  the  general  appearance  of  the  patient, 
then  the  shape  of  the  head  and  face;  next  the 
eyes,  as  to  whether  they  are  large  and  prominent,  or 
small  and  sunken-looking.  Listen  patiently  to  the 
sufferer's  complaints,  and  having  submitted  to  this 
ordeal,  test  the  acuteness  of  vision  of  each  eye 
separately,  and  afterwards  together,  writing  down 
the  result,  remembering  always  to  commence  with 
convex  glasses.  Then  place  the  near  type  in  the 
patient's  hand,  noting  the  number  of  the  type  and 
the  distance  at  which  it  can  be  read.  Next  pass  on 
to  the  ophthalmoscope,  first  applying  the  "  retino- 
scopic  test,^^  then  the  "indirect  examination,"  and 
finally  the  "  direct  method,"  first  at  a  distance,  and 
then  close  to  the  eye.  If  any  ametropia  exist,  the 
advisability  of  paralysing  the  accommodation  with  a 
mydriatic  must  be  considered. 

In  order  to  illustrate  this  method  of  examination,  I 
will  give  a  few  additional  cases. 

Case  23.  Hypermetropia. — E.  M — ,  a  young  woman, 
a   book-folder,    aet.    17,    suffering   from   tinea    tarsi, 


CASES  245 

complains  tliat  her  eyes  get  very  tired  at  niglit,  so 
mucli  so,  in  fact,  tliat  she  is  unable  to  read  for  any 
length  of  time.  Her  general  appearance  is  healthy, 
and  nothing  special  is  noticed  about  her  face,  except 
that  the  eyes  are  small.  The  acuteness  of  vision  for 
both  eyes  is  normal.  On  placing  +  1  D.  in  front  of 
the  right  eye,  |-  is  seen  more  distinctly  than  without, 
with  +  2  D.  -|  is  still  read,  with  +  2*5  D.  vision  is 
not  so  good  j  the  same  result  is  obtained  with  the 
left  eye.  +  2  D.  for  each  eye  is  the  strongest  convex 
glass  with  which  |-  can  be  read,  and  is  therefore  the 
measure  of  her  Hm. ;  on  trying  the  two  eyes  together 
+  2"5  D.  still  gives  |-.     We  record  it  thus : 

R.V.  fHm.  2D.  =  |^)  « 

6  b  ^Hrn.  2oD.  =  f. 

L.V.  |Hm.  2D.  =  |) 

On  placing  the  patient  in  the  dark  room,  and 
directing  her  to  look  at  some  distant  object  or  at  a 
black  wall,  so  as  to  relax  as  much  as  possible  the 
accommodation,  Avith  the  plane  mirror  the  shadow 
we  perceive  moves  slowly  with  the  mirror.  We  put 
+  2  D.  in  a  spectacle  frame,  in  front  of  the  eye ;  the 
shadow  is  more  distinct,  and  moves  more  quickly. 
We  try  stronger  glasses,  and  then  find  that  +  3*5  D. 
is  the  highest  with  which  we  still  get  a  shadow 
moving  with  the  mirror.     Both  eyes  are  alike. 

Next  examine  Avith  the  ophthalmoscope.  By  the 
indirect  method  the  disc  becomes  smaller  on  with- 
drawing the  objective  from  the  eye.  With  the 
mirror  alone  at  a  distance,  we  see  an  image  of  the 
disc  which  moves  with  the  observer's  head,  proving 


246  THE    REFRACTION   OF    THE    EYE 

the  image  to  be  an  erect  one.  With  the  direct 
method  the  disc  is  not  seen  well,  unless  we  put  in  force 
our  own  accommodation;  with  our  accommodation 
suspended,  we  turn  the  wheel  of  the  ophthalmoscope 
so  as  to  bring  forward  convex  glasses ;  the  clearness 
of  the  fundus  is  improved;  +  4  D.  is  the  strongest 
convex  glass  with  which  the  details  can  be  distinctly 
and  clearly  seen  by  myself. 

We  might  be  satisfied  with  this  result,  assuming 
4  D.  to  be  the  amount  of  total  hypermetropia,  but  in 
young  people  it  is  much  more  satisfactory  to  be  able 
to  record  once  and  for  all  the  total  hypermetropia 
beyond  doubt.  Atropine  (grs.  iv  to  5J)  is  therefore 
ordered,  one  or  two  drops  to  be  placed  in  both  eyes 
three  times  a  day  for  four  days,  warning  her  that 
she  will  be  unable  to  see  well,  and  that  the  pupils 
will  be  dilated  during  their  use.  We  also  recommend 
a  shade  to  be  worn  to  protect  the  eyes  from  the  light. 

Gn  her  return  she  reads  only  g-^  with  each  eye, 
and  she  now  requires  +  5  D.  to  enable  her  to  read 
-J.  We  also  find  with  retinoscopy  that  +  5  D.  is 
the  strongest  glass  with  which  we  get  a  shadow 
moving  with  the  mirror. 

Our  patient,  therefore,  has  a  total  hypermetropia 
of  5  D.,  two  dioptres  of  which  were  manifest,  and 
three  latent.  For  work  and  reading  we  order  her 
spectacles  +  3  D.  At  present  she  will  not  require 
them  for  distance.  About  thirty  she  will  probably 
require  her  glasses  increased  to  +  4  D. ;  about  forty 
she  may  be  able  to  bear  her  full  correction,  and  may 
then  begin  to  wear  them  constantly. 


CASES  247 

We  must  remember  that  when  atropine  has  been 
used  it  is  necessary  to  take  off  '50  D.  from  the  measure- 
ments thus  found. 

Case  24.  Myopia. — A  young  man,  set.  20,  next 
presents  himself.  He  has  a  long  intellectual  face 
with  prominent  nose ;  the  palpebral  apertures  are 
wide  ;  and  on  directing  him  to  look  inwards  as  much 
as  possible,  the  eyeballs  seem  elongated  in  the  antero- 
posterior diameter. 

His  eyes,  he  says,  are  excellent,  but  he  is  unable  to 
recognise  people  as  well  as  formerly.  We  test  the 
acuteness  of  vision,  and  find  that  he  reads  -^^  with 
each  eye.  Convex  glasses  make  even  that  line  in- 
distinct, our  patient  is  probably  myopic.  We  place 
in  his  hand  the  near  type,  and  he  reads  No.  1  at 
once  and  easily.  The  farthest  point  at  which  he  can 
read  it  is  25  cm.  (-y/  =  4  D.)  ;  -  4  D.  should  be  the 
measure  of  his  myopia.  We  try  —  4  D.,  directing 
him  again  to  look  at  the  distant  type.  He  reads  with 
each  eye  |- ;  we  reduce  the  glass  to  find  the  weakest 
with  which  he  reads  the  same,  and  with  —  3*5  D.  he 
reads  it,  though  hardly  so  well ;  with  —  3D.  he  reads 
only  f;  -  3*5  D.  is  therefore  the  measure  of  his 
myopia,  and  we  record  it  thus  : 

E  Y  _6_-3-5D.=  6 
L.V.3%-3-5D.  =  f. 

If  we  employ  retinoscopy  —  3*5  D.  is  the  concave 
glass  which  neutralises  the  shadow. 

We  next  subject  the  eye  to  the  indirect  ophthalmo- 


248  THE    REFRACTION    OF    THE    EYE 

scopic  examination.  The  image  of  the  disc  becomes 
larger  on  placing  the  objective  near  the  eye  and 
gradually  withdrawing  it^  and  in  addition  we  see  also 
a  slight  myopic  crescent  on  the  apparent  inner  side  of 
the  disc.     From  this  case  disc  No.  1  was  drawn  (p.  147). 

With  the  mirror  alone  at  a  distance  from  the  eye 
the  disc  cannot  be  well  seen,  because  in  our  case  the 
aerial  image  will  be  formed  about  25  cm.  in  front  of 
the  patient's  eye.  To  enable  us  to  see  this  aerial 
image  it  is  necessary  we  should  be  some  30  cm.  away 
from  it :  so  that  we  should  require  to  be  25  +  30 
=  55  cm.  from  the  observed  eye,  and  at  that  distance 
the  illumination  will  be  very  weak. 

With  the  direct  method  the  details  appear  blurred 
until  we  put  on  a  concave  glass  by  turning  the  wheel 
of  our  refracting  ophthalmoscope.  The  weakest  con- 
cave glass  with  which  we  are  able  to  see  the  details 
of  the  fundus  clearly  is  the  measure  of  the  myopia. 
Thus  we  have  four  distinct  plans  of  measuring  our 
case  of  myopia  : 

1st.  The  farthest  distance  at  which  the  near  type  is 
read,  25  cm.  {-^^  =  4  D.). 

2nd.  The  weakest  concave  glass  which  gives  the 
greatest  acuteness  of  vision. 

3rd.  The  weakest  concave  glass  with  which  we  get 
a  retinoscopic  shadow  moving  with  the  plane  mirror. 

4th.  The  weakest  concave  glass  Avith  which  the 
details  of  the  fundus  can  be  distinctly  seen  by  the 
direct  method. 

Should  any  of  these  results  vary  much,  we  should 
suspect  that  the  myopia  is  increased  by  spasm  of  the 


CASES  249 

accommodation,  and  we  atropize  the  patient  in  tlie 
manner  before  described,  and  at  the  end  of  fonr  days 
go  over  tlie  ground  again,  remembering  that  when 
atropine  has  been  used,  it  is  necessary  to  add  on 
about  —  '5  D.  to  the  glass  found,  because  the  ciliary 
muscle  is  probably  never  so  completely  relaxed  as 
when  it  is  under  the  influence  of  atropine. 

Having  found,  then,  that  our  patient's  myopia 
amounts  to  —  3*5  D.,  we  give  spectacles  of  that  focus 
for  constant  use.  In  addition  to  ordering  spectacles, 
we  give  him  also  some  very  important  general  direc- 
tions :  he  must  always  hold  his  book  or  work  35  cm. 
away,  bring  the  work  to  his  eyes,  and  not  his  eyes  to 
the  work ;  writing  should  be  done  at  a  sloping  desk, 
he  should  sit  with  his  back  to  the  window,  so  that  the 
light  comes  over  his  left  shoulder  on  to  his  work,  and 
do  as  little  near  work  as  possible  by  artificial  light. 

Case  25.  Hypermetropia  and  Presbyopia. — A  gentle- 
man, 83t.  56,  comes  with  the  complaint  that  he  cannot 
see  to  read  as  comfortably  as  formerly,  though  he 
sees  distant  objects  well.  We  try  his  acuteness  of 
vision,  and  find  that  he  reads  f  badly.  With  +  1  D. 
he  sees  much  better,  reading  some  of  the  letters  of  -f. 
We  then  try  +  1"5  D.,  and  these  he  rejects.  Hence 
we  conclude  that  he  has  Hm.  1  D.  We  know  from  his 
age  that  he  will  also  be  presbyopic  3  D.,  and  we  add 
on  to  this  +  1  D.  for  his  hypermetropia,  directing 
him  to  read  the  newspaper  with  +  4  D.  for  half  an 
hour.  He  thinks  these  rather  strong  for  him,  as  they 
make  his  eyes  ache.  With  +  3*5  D.  he  feels  quite 
comfortable,   and   we    therefore  give  him   +  3*5  D., 


250  THE    REFRACTION    OP    THE    EYE 

telling  him  that   lie  may  require  tliem  clianged  for 
slightly  stronger  ones  in  about  five  years. 

Case  26.  Paralysis  of  the  Accommodation. — Kate 
L — ,  £et.  12,  has  been  very  ill  from  diphtheria,  but  is 
now  much  better.  She  complains  that  she  is  unable 
to  read  or  work,  though  able  to  see  distant  objects 
well.  The  pupils  are  very  large,  and  act  badly  to 
light.  Hence  we  suspect  paralysis  of  the  accommo- 
dation. We  test  her  acuteness  of  vision,  and  she  sees 
|-  with  each  eye ;  on  trying  convex  glasses  '5  D.  she 
still  reads  f,  but  1  D.  she  rejects.  Our  diagnosis 
is  therefore  confirmed.  We  next  find  the  weakest 
glass  with  which  she  is  able  to  read,  weakest  because 
we  are  anxious  to  encourage  the  ciliary  muscle  to  act, 
since  by  replacing  it  entirely  we  should  prolong  the 
patient^s  recovery. 

The  glasses  must  be  changed  for  weaker  ones  as 
the  ciliary  muscles  recover  tone. 

We  saw  that  she  had  a  slight  amount  of  hyperme- 
tropia,  and  also  that  there  was  some  accommodation 
left,  enough  at  least  to  correct  this,  otherwise  she 
could  not  have  read  f  without  +  '5.  A  tonic  con- 
taining iron  and  strychnine  was  also  prescribed. 

Case  27.  Anisometropia. — A  young  woman,  ^t.  20, 
has  never  seen  well,  either  at  a  distance  or  near  at 
hand ;  has  tried  spectacles  of  all  sorts,  but  never  been 
able  to  find  any  that  suited  her.  The  eyes  look 
somewhat  irritable,  but  there  is  nothing  conspicuous 
about  their  size  or  shape.  There  is  some  want  of 
symmetry  about  the  face,  the  nose  being  deviated 
from  the  median  line  slightly  to  the  left. 


CASES  251 

We  first  try  the  acuteness  of  vision  of  tlie  riglit  eye. 
She  reads  y^,  and  with  +  1  D.  vision  is  somewhat 
improved;  with  +  1*5  D.  it  is  made  worse.  Still 
armed  with  +  1  D.  we  direct  the  patient  to  look  at 
the  fan  of  radiating  lines  (Fig.  85).  She  sees  plainly 
the  horizontal  lines,  whilst  all  the  others  are  more  or 
less  indistinct,  the  vertical  line  most  so ;  still  looking 
at  the  horizontal  line,  we  alternately  hold  in  front  of 
+  1  D.,  which  is  before  the  eye  under  examination, 
+  '25  D.,  which  makes  it  worse,  then  —  '25  D.,  which 
she  says  at  once  makes  it  perfectly  clear  and  distinct. 
We  therefore  put  down  +  '75  as  the  correction  for 
the  vertical  meridian,  and  pass  on  to  the  horizontal. 
Our  patient  is  directed  to  look  steadily  at  the  vertical 
line.  We  try  convex  glasses,  these  improve  it,  +  3  D. 
making  it  quite  clear ;  a  stronger  glass  than  this 
renders  it  slightly  indistinct.  It  is  evident,  there- 
fore, that  her  horizontal  meridian  is  hypermetropic 
+  3  D.  We  put  up  the  correction  found,  +  '75  D. 
sp.,  +  2*25  D.  cylinder  axis  vertical,  and  direct  her 
again  to  look  at  the  distant  type ;  -|  is  read,  though 
with  some  difficulty.  This  result  is  not,  however, 
reliable,  and  we  proceed  to  confirm  it  by  retinoscopy, 
obtaining  +  2  D.  for  the  vertical,  and  +  4  D.  for  the 
horizontal  meridians.  On  trying  this  correction, 
however,  the  vision  is  not  so  good.  We  now  test  the 
acuteness  of  vision  in  the  left  eye ;  she  sees  -g^,  and 
neither  convex  nor  concave  glasses  improve  it.  On 
looking  at  the  fan  of  radiating  lines,  all  seem  indis- 
tinct, and  having  thus  far  no  data  to  go  upon,  we, 
instead  of  wasting  time,  at  once  pass  on  to  retino- 


252  THE    REFEACTION    OP    THE    EYE 

scopy.  We  get  oblique  shadows^  the  horizontal 
moving  against  the  mirror  and  the  vertical  with  it : 
here^  then^  is  a  case  of  mixed  astigmatism,  We  find 
out  that  —  2  D.  is  the  correction  for  the  horizontal 
meridian  and  +  3  D.  for  the  vertical^  the  degree  of 
obliquity  being  about  25°.  This  result  is  noted  down 
thus  : 


3D. 


2D. 


We  therefore  place  in  a  spectacle  frame  +  3  D. 
spherical^  combined  with  -  5  D.  cylinder^  axis  devi- 
ating outwards  from  the  vertical  25°.  With  this  cor- 
rection the  patient  at  once  reads  yg"-  ^^  ^^®  ^^^^  ^^ 
be  satisfied  with  this  result^  but  give  the  patient  a 
solution  of  sulphate  of  atropine^  grs.  iv  to  3J>  "^^ith 
directions  to  come  again  in  four  days.  At  the  end  of 
that  time  she  returns^  and  we  find  with  retinoscopy — 


R. 


2-5  D. 
+  4-5  D. 


The  right  eye  with  this  correction  reads  ■§-  readily, 
and  the  left  also  -J,  but  rather  slowly.  This  result  is 
very  satisfactory.  At  the  end  of  a  week,  when  the 
patient  has  recovered  from  the  eifects  of  the  atropine, 
the  results  were  confirmed  before  ordering  spectacles; 
then  for  the  right  eye  the  best  vision  was  obtained  with 


CASES  253 

+  1*5  sp.  c:;  +  2  D.  cy,  axis  vertical  (|-) ;  and  for  the 
left  +  3  D.  spherical  o  -  5  D.  cylind.  axis  70°  (f). 
Spectacles  of  this  strength  were  therefore  ordered, 
and  the  patient  directed  to  wear  them  constantly. 

Case  28.  Anisometro'pia. — Jane  W — _,  set.  30,  pre- 
sents herself,  complaining  that  the  sight  in  her  left 
eye  has  been  gradually  getting  dim  for  some  months. 
She  is  a  small,  healthy-looking  woman,  with  nothing 
characteristic  in  her  appearance.  We  test  the  acute- 
ness  of  vision  : 

Right  f  Hm.  1  D.  =  f . 

Left  -yej  ^^^^  improved  with  spherical  glasses. 

We  try  retinoscopy,  but  the  pupils  are  so  small 
that  the  result  is  not  very  satisfactory.  One 
can,  however,  make  out  in  the  left  eye  a  shadow 
moving  with  the  mirror  in  the  horizontal  meridian, 
which  +  2  D.  over-corrects,  +  TS  D.  being  the 
highest  glass  with  which  we  get  a  shadow  moving 
with  the  mirror ;  the  vertical  meridian  appears  emme- 
tropic. There  is,  therefore,  no  doubt  that  the  de- 
fective vision  in  this  eye  is  due  to  astigmatism.  The 
patient  complains  that  the  examination  has  made  her 
eyes  ache,  so  we  do  not  proceed  further,  but  order  a 
solution  of  hydrobromate  of  homatropine  (2  grs.  to 
the  5J)  to  be  used  every  three  hours,  and  direct  her 
to  come  again  on  the  following  day.  Then  the  result 
with  retinoscopy  is — 

+  1-5  D.                             +  -SOD. 
E. +  1-5  D.         L. +  2  D. 


254  THE    EEFEACTION    OF    THE    EYE 


We  try  this  at  the  test  type. 
R.  2^  +  1-5  D.  =  f. 

I^    6    g+    -SD.  sp.  ^6 

•  3  6     + 1-5  D.  cy.  axis  vert.     6  * 

We  make  a  slight  deduction  from  the  sphere  in 
each  case  for  the  homatropine^  and  order  for  constant 
use — 

E.  +  1  D.  sph. 

L.  +  1*5  D.  cy.  axis  vert. 

Case    29.    Presbyopia. — John    G — ,     set.    50,    has 

always    enjoyed    good    sight ;    he    still    sees    distant 

objects  well,  but   finds    some    difficulty   in   reading, 

especially  during  the  evenings. 

R.V.f,  noHm. 

L.y.f,  noHm. 

We  try  him  with  +  2  D.  for  reading,  and  with 
these  he  sees  perfectly ;  this,  therefore,  is  a  simple 
case  of  presbyopia,  requiring  a  pair  of  folders  +  2  D. 
for  reading,  writing,  etc. 

Case  30.  Hypermetropia  and  Presbyopia. — Mr.  K — , 
set.  60,  sees  badly  both  near  and  distant  objects ; 
he  wears  +  4  D.  for  reading,  but  they  are  not  com- 
fortable. 

E.V.g^Hm.  3D.  =  f . 

L.V.3%Hm.3D.=f. 

He  therefore  wants  +  3  D.  for  distance ;  and  to 
find  the  glass  he  will  require  for  reading,  it  is  neces- 
sary to  add  on  to  this  distance  lens  the  glass  he 
would  require  for  presbyopia  if  he  were  an  emme- 
trope,  viz.  +  4  D.     We  therefore  try  him  with  +  7  D., 


CASES  255 

but  these  make  his  eyes  ache;  we  next  try  +  6*5  D., 
and  with  these  he  sees  comfortably. 

This   patient_,   then^   requires   two   pairs   of    spec- 
tacles^— 

+  3  D.  for  distance  ; 

+  6"5  D.  for  reading,  &c. 
Case  31.  Myopia  and  Presbyopia. — Mrs.   C — ,  aet. 
55,  complains  that  her  eyes  become  tired  at  night; 
she  has  tried  several  pairs  of  spectacles,  but  without 
finding  any  that  exactly  suit  her. 


L.V.-A--2D. 


6 


36    ^^-"g- 

Our  patient  requires,  therefore,  this  correction  for 
distance,  but  she  also  wants  spectacles  for  reading 
and  near  work ;  an  emmetrope  of  fifty-five  requires 
presbyopic  glasses  +  3  D. ;  she  is,  however,  a  myope 
of  2  D.,  so  we  have  to  deduct  this  from  the  presbyopic 
glass  (  (+  3D.)  +  (-  2D.)=  +  1  D.),  and  try  the 
+  1  D.  for  reading.  With  these  she  is  able  to  read 
the  smallest  type  comfortably ;  we  therefore  pre- 
scribe two  pairs  of  spectacles, — 

-  2  D.  for  distance  ; 

+  1  D.  for  reading. 

Case  32.  Myopia. — Annie  C — ,  aet.  19,  came  because 
she  was  unable  to  see  distant  objects. 
E.V. -2^-3-5  D.  =  f. 
L.Y.  #i--2-5D.  =  #. 


After  using  atropine — 


B.V./g-8D.=f, 
L.V.-,^-2D.  =  f. 


256  THE    REFRACTION    OF    THE    EYE 

Ordered  spectacles  for  distance  R.  —  3  D.^  L.  —  2 
ID.,  with  directions  to  present  herself  again  in  six 
months,'  when,  should  the  myopia  have  increased, 
or  should  she  complain  of  asthenopia,  it  may  be 
necessary  to  prescribe  spectacles  for  constant  use. 

Case  33.  Sim'ple  Myoinc  Astigmatism. — Thomas 
J — ,  ast.  18,  sees  rather  badly  both  near  and  distant 
objects. 

E.V.  -j^2^  not  improved  with  spheres  ;  with  pin-hole  =  ^. 
L.V.  -j^2''  ^ot  improved  with  spheres  ;  with  pin-hole  = -I". 

After  atropine  had  been  used  for  four  days  retino- 

scopy  gave — 

,  +  1  D.  [+  1  D. 

R.     ^ Em.  L.      ^ -Em. 

R.  +  1  D.  cy.  axis  horiz.=^. 
L.  +  1  D.  cy.  axis  horiz.  =  ^. 
After  the  atropine  has  passed  off — 
R.  —  l  D.  cy.  axis  vert.  =  ^- 
L.  — 1  D.  cy.  axis  vert.  =  ^. 
This  correction  was  given  for  constant  use. 
Case  34.       Gompoiond   Myopic  Astigmatism. — Miss 
W — ,  set.  13,  has  seemed  short-sighted  for  the  last  year 
or  two.     Mother  and  father  both  have  good  sight. 

The  pupils  are  large,  so  that  retinoscopy  can  be 
easily  carried  out. 

10  D. 


!  - 10  D. 
1 
R. 6  D. 


-6D. 


CASES 

T>y  gZJ^J^L^-       .    ,      .    =.-,%- and  2  letters  of -,%, 
-K"  V  •  c  _  4  D.  cy.  axis  lioriz.      i  8  1-2 

.-YD.  sp.  6 


257 


-  6  D.  sp 

c 

L.V 


-3D.  cy.  axis  horiz.      1  2  • 

On  examination  of  the  eyes  with  the  ophthal- 
moscope the  choroid  is  found  to  be  exceedingly  thin, 
there  is  a  large  crescent  in  both  eyes,  and  in  the  right 
are  three  or  four  patches  of  choroiditis,  with  one 
haemorrhage  near  the  macula. 

The  patient  was  ordered  the  full  correction  for 
distance,  and  advised  to  do  no  reading,  writing,  or 
near  work  for  six  months,  then  to  return  for  inspec- 
tion; she  was  also  recommended  to  spend  as  much 
of  her  time  as  possible  in  the  open  air,  and  a  mixture 
containing  syrup  of  the  iodide  of  iron  was  pre- 
scribed. 

Case  35.  Concomitant  Squint. — George  W — ,  £et.  5, 
has  squinted  inwards  for  the  last  three  months.  On 
covering  the  non-squinting  eye  and  directing  the 
little  boy  to  look  at  the  finger  held  a  short  distance 
from  him,  the  deviating  eye  immediately  righted 
itself  and  fixed  the  finger,  the  covered  eye  at  the 
same  time  turning  in.  We  prescribed  a  solution  of 
sulphate  of  atropine  to  be  applied  to  both  eyes,  and 
at  the  end  of  a  week  the  patient  is  brought  back :  the 
squint  is  now  much  less  apparent,  and  with  retino- 
scopy  we  find  3'5  D.  of  hypermetropia  in  each  eye. 
The  direct  examination  gives  the  same  result.  We 
order  our  patient  spectacles  +  3  D.  to  be  worn  con- 
stantly. 

Case    36.  Aphakia. — Thomas    B — ,  ast.    50,   game- 

17 


258  THE    EEFRACTION    OF    THE    EYE 

keeper.  Had  the  right  lens  removed  for  cataract 
nine  months  ago,  and  last  week  the  opaque  capsule 
remaining  was  needled. 

E.V.  c  +  11  D.  =  f,  and  with  +  14  D.  No.  1  of 
the  near  type  was  read  with  comfort;  the  patient 
was  therefore  ordered  the  following  spectacles : 

+  11  D.  for  distance; 
+  14  D.  for  near  Avork. 

These  were  arranged  in  a  reversible  frame,  so  that 
either  glass  could  be  brought  in  front  of  the  right  eye 
as  occasion  required. 


APPENDIX  259 


APPENDIX 

In  tlie  metrical  system  tlie  unit  of  length  is  a 
metre^  equal  to  100  centimetres^  1000  millimetres,  or 
40  English  inches ;  so  that  1  inch  is  equal  to  2  J  cen- 
timetres. A  lens  of  1  metre  focus  is  called  a  dioptre, 
a  lens  of  ^  a  metre  (50  cm.)  is  2  D.,  -^-^  of  a  metre 
(10  cm.),  10  J).,  etc. 

In  the  old  system  the  lenses  were  numbered 
according  to  their  focal  length  in  inches,  a  lens  of 
1-inch  focus  being  the  unit ;  a  lens  of  2-inch  focus 
was  expressed  by  the  fraction  ^,  one  of  10-incli  focus 
yij,  and  so  on.  If  we  wish  to  convert  a  dioptric 
measurement  into  the  corresponding  inch  measure- 
ment of  the  old  system,  we  have  only  to  remember 
that  the  unit  1  metre  =  40  English  inches,  so  that  a 
glass  of  1  D.  =  4^5-  in  the  old  system,  2  D.  ='^V  —  ~2V^ 
5  D.  =  -^^  =  "I",  and  so  on. 

The  table  on  the  next  page  gives  approximately 
the  equivalent  of  each  dioptre  or  part  of  a  dioptre  in 
English  and  French  inches,  and  their  focal  length  in 
centimetres. 


260 


THE    REFRACTION    OF    THE    EYE 


Dioptres. 

English  inches. 

French  inches. 

Centimetres. 

•25 

160 

146 

400 

•50 

80 

73 

200 

•75 

52 

50 

130 

!• 

40 

36 

100 

1-25 

31 

29 

77 

1-50 

26 

24 

65 

1-75 

22 

21 

55 

2- 

20 

18 

50 

2-25 

17 

16 

43 

2-50 

16 

15 

40 

2-75 

14 

13 

35 

3- 

13 

12 

33 

3-50 

11 

10 

27 

4- 

10 

9 

25 

4-50 

9 

8 

22 

5- 

8 

7 

20 

5-50 

7 

6.^ 

17 

c- 

6i 

6 

16 

7- 

6 

5 

.15 

8- 

5 

4| 

12^^ 

9- 

4| 

4 

11 

lo- 

4 

31 

10 

ll- 

3.^ 

H 

9 

12- 

H 

3 

8 

]3- 

3 

2f 

7^ 

14- 

2| 

2^ 

7 

15- 

2* 

2i 

eh 

16- 

2i 

2i 

6 

18- 

2i 

2 

5i 

20- 

2 

n 

5 

APPENDIX  261 

Regulations  for  Candidates  for  Commissions  in  the 
Army 

A  candidate  must  be  able  to  read  at  least  -f-^  with 
each  eye  separately  without  glasses,  and  this  must 
be  capable  of  correction  with  glasses  up  to  |^  in  one 
eye  and  -^-^  in  the  other ;  he  must  also  be  able  to  read 
No,  1  of  the  near  type  with  each  eye  without  the  aid 
of  glasses. 

Squint,  colour-blindness,  or  any  serious  disease  of 
the  eye  renders  the  candidate  ineligible. 

Navy 

A  candidate  must  be  able  to  read  |^  with  each  eye, 
and  the  near  type  at  the  distance  for  which  it  is 
marked,  without  glasses. 

Colour-blindness,  squint,  or  any  disease  of  the  eye 
disqualifies. 

Indian  Civil  Service 

A  candidate  must  be  able  to  read  |-  with  one  eye 
and  |-  with  the  other,  with  or  without  correcting 
lenses. 

Any  disease  of  the  fundus  renders  the  candidate 
ineligible.  Myopia,  however,  with  a  posterior  sta- 
phyloma, may  be  passed  if  the  ametropia  do  not 
exceed  2*5  D.,  and  the  candidate  has  a  visual  acute- 
ness  equal  to  that  stated  above. 

Indian  Medical  Service 
The  candidate  must  have  a  visual  acuteness  of  -|  in 
one  eye  and  jV  "^  ^^^    other.      Hypermetropia    and 


262  THE    REFEACTION    OF    THE    EYE 

myopia  must  not  exceed  5  D.,  and  then  witli  the 
proper  correction  the  vision  must  come  up  to  the 
above  standard. 

Astigmatism  does  not  disqualify  a  candidate,  pro- 
vided the  combined  spherical  and  cylindrical  glass 
does  not  exceed  5  D.,  and  the  visual  acuteness  equals 
|-  in  one  eye  and  -^^  in  the  other.  Colour-blindness, 
ocular  paralysis,  or  any  active  disease  of  the  fundus 
renders  the  candidate  ineligible. 

A  nebula  of  the  cornea  will  not  disqualify  the 
candidate  if  he  is  able  to  read  -f-^  with  this  eye  and  -| 
with  the  other. 

Public   Worlcs 

Candidates  for  the  departments  of  Public  Works, 
Survey,  Forest,  Telegraph,  Railways,  Factories,  and 
Police  of  India  must  pass  the  following  eyesight  tests. 

If  myopic,  the  defect  must  not  exceed  2*5  D.,  and 
with  this  glass  the  candidate  must  read  |-  in  one  eye 
and  f  in  the  other. 

If  myopic  astigmatism  is  present,  the  vision  must 
reach  the  above  standard  with  correcting  glasses,  and 
the  combined  spherical  and  cylindrical  glass  must 
not  exceed  2-5  D. 

In  hypermetropia  and  hypermetropic  astigmatism 
an  error  of  4  D.  is  permissible  provided  that  with  this 
glass  |-  is  read  with  one  eye,  and  -g-  with  the  other. 

A  corneal  nebula  with  vision  of  j~  ^^^^  tt  ^^^  ^^^® 
other  eye  will  not  disqualify  the  candidate. 

Colour-blindness,  any  disease  of  the  eye,  or  paralysis 
of  one  of  the  muscles  of  the  globe,  will  disqualify. 


APPENDIX  263 

English  Railways 

There  is^  unfortunately,  no  uniform  standard  for 
our  railways  ;  each  company  has  its  own  standard,  in 
many  cases  a  very  low  one;  every  engine-driver 
should  have  at  least  jj  in  each  eye  without  glasses, 
and  normal  colour  vision. 


TEST  TYPES  265 


TEST    TYPES 


No.  1.  25cm.*=  /(f.. 


1  danger  of  breaking  do' 


We  passed  that  Act  because  we  thought 
,  was  not  then  sofficicntly  secured.  Yet 
icnvv)  than  it  will  be  if  this  House  takes 


No.  2.  1  J.  33cm.  =  /J^ 

on  itself  to  be  the  supreme  criminal  judicature  in  political  cases."  Warm  eulogies  were  pronounced  on  the 
ancient  national  mode  of  trial  by  twelve  good  men  and  true ;  and  indeed  the  advantages  of  that  mode  of  trial  in 
political  cases  are  obvious.  The  prisoner  is  allowed  to  challenge  any  number  of  jurors  with  cause,  and  a 
considerable  number  without  cause.  The  twelve,  from  the  moment  at  which  they  are  invested  with  their  short 
magistracy,  till  the  moment  when  they  lay  it  down,  are  kept  separate  from  the  rest  of  the  community.  Every 
precaution    is   taken   to  prevent  any  agent  of    power   from    soliciting    or    corrupting  them.      Every   one    of    them 


No.  3.  2  J.,  '50  Sn.t  50cm.  x^^^ 

must  hear  uvery  word  of  the  evidence  and  every  argument  used  on  either  side.  The  case  is  then  summed  up 
by  a  judge  who  knows  that,  if  he  is  guilty  of  partiality,  he  may  be  called  to  account  by  the  great  inquest  of 
the  nation.  In  the  trial  of  Fenwick  at  the  bar  of  the  House  of  Commons  all  these  securities  were  wanting. 
Some  hundreds  of  gentlemen,  every  one  of  whom  had  much  more  than  half  made  up  his  mind  before  the  case 
was  opened,  performed  the  functions  both  of  judge  and  jury.     They  were  not  restrained,  as  a  judge  is  restrained, 


*  The  number  indicates  the  distance  at  which  the  type  should  be 
seen  by  a  normal  eye. 

t  Corresponding  Jaeger  and  Snellen  type. 


266  TEST     TYPES 


No.  4.  5  J.,  -75  Sn.  75cm.  =:J(\ 


I>y  the  sense  of  responsibility ;  for  who  was  to  punish  a  Parliament  ?  They  were 
not  selected,  as  a  jury  is  selected,  in  a  manner  which  enables  the  culprit  to  exclude 
his  personal  and  political  enemies.  The  arbiters  of  his  fate  came  in  and  went  out 
as  they  chose.     They  heard  a  fragment  here  and  there  of  what  was  said  against  him, 


No.  5.  6  J.,  1  Sn.  lm.:3ff^ 

and  a  fragment  here  and  there  of  what  was  said  in  his  favour.  During 
the  progress  of  the  bill  they  were  exposed  to  every  species  of  influence. 
One  member  was  threatened  by  the  electors  of  his  borough  with  the 
loss  of   his   seat :   another   might  obtain  a   frigate   for  his  brother  from 


No.  6.  8  J.,  1-25  Sn.  l-25m.r^^ 

Russell ;  the  vote  of  a  third  might  be  secured  by  the 
caresses  and  burgundy  of  Wharton.  In  the  debates  arts 
were  practised  and  passions  excited  which  are  unknown 
to  well- constituted    tribunals,    but    from   which    no    great 


No.  7.  10  J.,  1-5  Sn.  l-5m.-  ^ 

popular   assembly   divided   into   parties  ever  was  or 
ever  will  be  free.     The  rhetoric  of   one  orator  called 


TEST    TYPES  267 

No.  8.  12  J.,  2  Sn.  2m.  <iO 

forth  loud  cries  of  *'  Hear  him."     Another 
was  coughed  and  scraped  down.     A  third 

No.  9.  14  J.  2-5m. 

Spoke  against  time  in  order 
that    his   friends  who   were 

No.  10.  16  J.  3-5m. 

supping:  mi§:ht   come 

No.  11.  5m. 

in  to  divide.  If 

No.  12.  7m. 

prominent 


268 


TEST    TYPES 


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N 


N 

> 
O 

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N 

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INDEX 


Abducting'  pi-isms,  45 
Accommodation,  32,  187 

absolute,  40,  213 

amplitude  of,  37 

at  diiJerent  ages,  41 

binocular,  40 

diminution  of,  40,  187 

of  emmetropes,  37 

of  h.^-permetropes,  38 

of  myopes,  39 

paralysis  of,  196 

produced  by,  33 

range  of,  37 

relative,  40,  50 

spasm  of,  197 
Accommodative  asthenopia,  124,  132, 

226 
Acquired  hj'permetropia,  128 
Acuteness  of  vision,  29,  55 

in  hjqaermetropia,  126 

in  mj^opia,  148 

in  astigmatism,  165 

diminishes  with  age,  187 
Adducting  prism,  46 
Aerial  image,  73,  245 
Alternating-  strabismus,  206 
Amblyopia,  211 
Amblyoscope,  220 
Ametropia,  26 
Amplitude  of  accommodation,  37 

of  convergence,  44 
Anderson,  Dr.  Tempest,  182 
Angle  a,  41,  123,  145,  201 

V,  203 

metrical,  of  convergence,  44 

of  deviation,  9 

of  strabismus,  207 

principal,  9 

visual,  29,  56 
Anisometropia,  29,  184 

correction  of,  185 

treatment  of,  185 
Anterior  focal  point,  24 

focus,  10 
Aphakia,  132 

case  of,  256 

test  for,  132 

treatment  of,  133 
*i.pparent  strabismus,  123,  200 


Appendix,  259 

Army,  regulations  for,  261 

Asthenopia,  124,  224 

accommodative,  124,  132,  226 

muscular,  146,  228 

retinal,  234 

retinal  veins  in,  235 
Astigmatism,  29,  156 

causes  of,  164 

compound  hypermetropic,  161 

compound  myopic,  161 

estimation  of,  167 

irregular,  157 

mixed, 161 

principal  meridians  in,  158,  168 

regular,  157 

shape  of  disc  in,  171 

simple  hjqjermetropic,  161 

simple  myopic,  161 

svmptoms  of,  164 

treatment  of,  176,  238 
Astigmatic  clock-face,  170 

fan, 171 
Asymmetry  of  cornea,  157 
Atropine,  60,  89,  126, 151, 170, 196 

in  astigmatism,  170 

in  myopia,  151 

in  retinoscopy,  89 

in  hypermetropia,  126 
Axial  line,  121,  138 

hypermetropia,  121 

mj'opia,  138 
Axis,  optic,  201 

principal,  9,  12 

secondary,  12,  15 

visual,  201 

B 

Bar  reading,  222 
Biconcave  lenses,  11,  16,  20,  238 
Biconvex  lenses,  11,  19,  238 
Bifocal  lens,  241 
Binocular  accommodation,  40 
vision,  211,  219 


Capsule  of  lens,  34 
Cardinal  points,  23 
Cataract,  132,  148,  164 
in  myopia,  148 


■'-fc^^-'-.  '^:3fi'> 


m 


$ 


INDEX 


Abducting-  prisms,  45 
Accommodation,  82,  187 

absolute,  40,  213 

amplitude  of,  37 

at  different  ages,  41 

binocular,  40 

diminution  of,  40,  187 

of  emmetropes,  37 

of  hypermetropes,  38 

of  myojies,  39 

paralysis  of,  196 

produced  by,  33 

range  of,  37 

relative,  40,  50 

spasm  of,  197 
Accommodative  asthenopia,  124,  132, 

226 
Acquired  hypermetropia,  128 
Acuteness  of  vision,  29,  55 

in  hj^permetropia,  126 

in  myopia,  148 

in  astigmatism,  165 

diminishes  with  age,  187 
Adducting  prism,  46 
Aerial  image,  73,  245 
Altei'nating  strabismus,  206 
Amblj^opia,  211 
Amblj^oscope,  220 
Ametropia,  26 
Amplitude  of  accommodation,  37 

of  convergence,  44 
Anderson,  Dr.  Tempest,  182 
Angle  a,  41,  123,  145,  201 

y,  203 

metrical,  of  convergence,  44 

of  deviation,  9 

of  strabismus,  207 

principal,  9 

visual,  29,  56 
Anisometropia,  29,  184 

correction  of,  185 

treatment  of,  185 
Anterior  focal  point,  24 

focus,  10 
Aphakia,  132 

case  of,  256 

test  for,  132 

treatment  of,  133 
^i-pparent  strabismus,  123,  200 


Appendix,  259 

Army,  regulations  for,  261 

Asthenopia,  124,  224 

accommodative,  124,  132,  226 

muscular,  146,  228 

retinal,  234 

retinal  veins  in,  235 
Astigmatism,  29,  156 

causes  of,  164 

compound  hypermetropic,  161 

compound  myopic,  161 

estimation  of,  167 

irregular,  157 

mixed,  161 

principal  meridians  in,  158,  168 

regular,  157 

shape  of  disc  in,  171 

simple  hypermetropic,  161 

simple  mj'opic,  161 

svmptoms  of,  164 

treatment  of,  176,  238 
Astigmatic  clock-face,  170 

fan, 171 
Asymmetry  of  cornea,  157 
Atropine,  60,  89,  126,  151, 170, 196 

in  astigmatism,  170 

in  mj^opia,  151 

in  retinoscopy,  89 

in  hypermetropia,  126 
Axial  line,  121,  138 

hypermetropia,  121 

myopia,  138 
Axis,  optic,  201 

principal,  9,  12 

secondary,  12,  15 

visual,  201 

B 

Bar  reading,  222 
Biconcave  lenses,  11,  16,  20,  238 
Biconvex  lenses,  11,  19,  238 
Bifocal  lens,  241 
Binocular  accommodation,  40 
vision,  211,  219 


Capsule  of  lens,  34 
Cardinal  points,  23 
Cataract,  132,  148,  164 
in  myopia,  148 


270 


INDEX 


Cases,  retinoscopy,  102,  244 

others,  244 
Centre  of  motion  of  the  eye,  24 

optical,  12 
Choroid,  thinning  of,  in  mj^opia,  147 
Ciliary  muscle,  fvinction  of,  34 

in  hypermetropia,  123 
in  myopia,  145 

body,  34 
Civil  Service,  regulations  for,  261 
Cohn,  142 
Cocaine,  90 

Compound  hyi)ermetropic    astig- 
matism, 161 

myopic  astigmatism,  161 

system,  points  of,  23 
Concave  lenses,  11,  16,  20,  238 

mirror,  in  retinoscopy,  100 
Concomitant  squint,  200 
Conjugate  focus,  4,  14,  136 
Conjunctiva,  126,  225 
Convergence,  41 

amplitude  of,  44 

insuflBciency  of,  229 

latent,  229 

metrical  angle  of,  44 

punctum  proximum  of,  44 

punctum  remotum  of,  44 

range  of,  44 

relative,  50 
Convergent  strabismus,  124,  208 
Cone,  56,  144 

of  Ught,  158 
Convex  lenses,  11, 19,  238 
Cornea,  22 

image  formed  on,  34 
Crescent,  myopic,  146 
Crystalline  lens,  33,  122,  148,  157 
Cylindrical  glasses,  32,  157,  173 


Decentering  lenseSj  194,  233 
Detachment  of  retina  in  myopia,  148, 

155 
Deviation,  angle  of,  9 

primary,  204 

secondary,  204 
Dioptre,  31,  259 
Dioptric  system,  31 
Diplopia,  42,  213 
Direct  ophthalmoscopic  examination, 

53,73 
Disc,  Placido's,  181 

shape  of,  in  astigmatism,  172 
Distant  type,  56 

Divergent  strabismus,  146,  208,  215 
Divergence,  appearance  of,  123,  203 

latent,  230 
Donders,  121,  188,  226 


Educational  treatment  of  squint,  219 
Elasticity  of  capsule,  34 
of  lens.  34,  40 


Elasticitj'  of  lens,  diminution  with  age, 
40,  187 

Elongation  of  eyeball  in  myopia,  138 

Emergent  ray,  7 

Emmetropia,'  26 

punctum  proximum  in,  28,  35 
punctum  remotum  in,  28,  35 

Erect  image,  53,  73 

Erismann,  142 

Eserine,  144,  197 

Esophoria,  230 

Exercises,  orthoptic,  220 

Exophoria,  230 

Eye,  21 

refracting  media  of,  22 
refracting  surfaces  of,  22 


Face,  asymmetry  of,  in  astigmatism, 
53,  164 

in  hjqiermetropia,  53,  124 

in  myopia,  53 
Far  point,  see  punctum  remotum,  28, 

35 
Focal  length,  31,  259 

interval,  160 

points,  24 
Focus,  anterior,  10 

conjugate,  4,  14,  136 

principal,  3,  5,  9,  14,  24 
Formation  of  images,  17 

by  the  eye,  25 
Fundus,  146 


Glaucoma,  130,  195 
Glasses,  237 

biconcave,  11,  16,  20,  237 

biconvex,  11,  19,  238 

cylindrical,  32,  157,  173 

orthoscopic,  194 

periscopic,  241 

prismatic,  241 

spherical,  31 

stenopaic,  157,  242 

tinted,  243 
Goggles,  243 

H 

Hereditary  tendency  in  myopia,  141 
Herring's  drop  test,  223 
Heterophoria,  229 
Homatropine,  90 
Homonymous  images,  231  * 
Hypermetropia,  26,  59,  117 

al)solute,  121 

acquired,  128 

amount  of,  126 

angle  a  in,  41,  123,  150,  201 

axial,  122 

causes  of,  121 

diagnosis  of,  126 

estimation  of,  59,  126 

facultative,  121 


INDEX 


271 


Hvpermetropia,  latent,  59,  127 

length  of  eyeball  in,  122 

manifest  59,  126 

oriorinal,  128 

relative,  121 

spectacles  for,  129,  237 

symptoms  of,  124 

tests  for,  126 

treatment  for,  129,  237 
Hjqjerme tropic  astigmatism,  simple, 
161 

compound,  161 
Hyperphoria,  230 


Images,  crossed,  65 

formation  of,  17 

homonymous,  65 

in  astigmatism,  76 

in  emmetropia,  75 

in  hypermetropia,  76 

in  myopia,  77 

on  cornea,  34 

on  lens,  34 

projected,  67 

real,  18 

virtual,  3,  19 
Indian  Services,  regulations  for,  261 
Indirect    ophthalmoscopic     examina- 
tion, 53,  66 
Insufficiency  of  convergence,  229 

test  for,  232 
Internal  recti,  146,  217 
Interval  of  Sturm,  160 

focal,  160 
Inverted  image,  25 

ophthalmoscopic  images,  53, 
66 
Inversion  of  images  by  lenses,  18 

by  the  eye,  25 
Iris  in  accommodation,  34 

in  hypermetropia,  123 

in  myopia,  146 
Irregular  astigmatism,  157 


Jackson,  Dr.,  99 

Jaeger,  test  type,  61 

Javal  and  Schiotz  ophthalmometer,  176 


Lachrymal  apparatus,  126 
Landolt,  154. 
Latent  convergence,  229 
deviation,  231 
divergence,  230 
hypermetropia,  59,  127 
Length  of  eyeljall,  22 
focal,  31 

in  hypermetropia,  122 
in  myopia,  138 
Lens,  crystalline,  35,  122,  148,  157 
Lenses,  11,  31,  237,  260 


Lenses,  biconcave,  11,  16,  20,  238 

Ijiconvex,  11,  19,  238 

bifocal,  241 

conjugate  focus,  4, 14,  136 

converging,  12 

cvlindrical,  32,  157,  171 

decentred,  194,  233 

diverging,  12 

foci  of,  14,  16 

images  formed  by,  17,  19 

influence  of,  on  the  size  of  the 
image,  238 

principal  focus,  5,  14,  24 

refraction  by,  11 

spherical,  31 

table  for  presbyopia,  191 
Light,  artificial,  82,  150 
Long  sight,  see  presbyopia,  28,  1S7 

M 

Macula,  29,'56, 147 
Haddock's  rod  test,  46,  229 
Manifest  hyiJermetropia,  59,  126 
Medium,  refraction  by,  7 
Meniscus,  11 
Metrical  angle,  44 

system  of  lenses,  31,  260 
Microphthalmos,  123 
Mirror,  concave,  for  retinoscopy,  100 

plane,  for  retinoscopy,  84 

reflection  by  a  plane,  2 
from  a  concave,  3 
from  a  convex,  6 
Mixed  astigmatism,  161 
Monolateral  strabismus,  206 
Movements  of  mirror  in  retinoscopy,  85 
Musca?.  volitantes,  145 
Muscle,  ciliary,  34,  123,  145 

iris,  34,  123,  146 
Muscular  asthenopia,  146,  228 
Myopia,  27,  134 

axial,  138 

causes  of,  138 

determining  causes,  141 

diagnosis  of,  148 

estimation  of  degree,  148 

formation  of  image  in,  136 

length  of  eyeball  in,  138 

ophthalmoscopic      appearances 
in,  146 

posterior  staphyloma  in,  138 

progressive,  135 

stationary,  152 

statistics  in,  142 

symptoms  of,  144 

treatment  for,  150 
Mj^opic  astigmatism,  161 

crescent,  146 

N 

Nagel  on  convergence,  42 
Navy,  regulations  for,  261 
Near  point  (punctum  proximum),  28, 
35 


272 


INDEX 


Negative,  angle  a,  203 

Nerve,  optic,  in  hypermetropia,  124! 

in  myopia,  147 
Nodal  points,  24 
Nordenson,  statistics  of,  178 


Objective  examination,  53 

Operative  treatment  of  squint,  223 

Optics,  Chap.  I 

Optic  axis,  201 

disc  in  myopia,  146 
nerve  in  hypermetropia,  124 
in  myopia,  147 

Optical  centre,  12 

Ophthalmo-dynamometer,  47 

Ophthalmological  Congress,  31 

Ophthalmometer  of  Javal  and  Schiotz, 
176 

Ophthalmoscope,  53,  66 

direct  examination,  66,  73 
indirect  examination,  66 

Ophthalmoscopic  appearances,  146 

Optometer  of  Tweedy,  178 
wire,  35 

Oris'inal  hjqjermetropia,  128 

Orthoptic  exercises,  219 

Orthoscopic  lenses,  194 


Paralysis  of  accommodation,  196 

causes  of,  196 

treatment  of,  197 
Pantoscopic  spectacles,  241 
Perimeter,  207 
Periodic  strabismus,  206 
Periscopic  lenses,  241 
Pin-hole  test,  54 
Placido's  disc,  181 
Plane  mirror,  84 
Points,  cardinal,  23 

nodal,  23 

principal,  23 
Position  in  myopia,  150 
Posterior  staphyloma,  138 
Pray,  test  letters  of,  168 
Presbyopia,  28,  187 

age  at  which  it  commences,  189 

definition  of,  188 

glasses  for,  190 

symptoms  of,  190 

table  for,  191 

treatment  of,  190 
Principal  angle,  9 

focus,  3,  5,  9,  14,  24 

points,  24 
Prismatic,  spectacles,  242 
Prisms,  8,  42,  240 

abducting,  45,  242 

adducting,  46,  242 

to  test  convergence,  42 
PriKmos])heres,  233 
Progressive  myopia,  135 


Protectors,  243 

Public  works,  regulations  for,  262 

Punctum  proximum,  28,  35 

in  emmetropia,  35 
in  hypermetropia,  38 
in  myopia,  39,  137 
remotum,  28,  35,  137 

m  emmetropia,  35 
in  hji)ermetropia,  38 
in  myopia,  137 
Pupil  in  accommodation,  34 
in  hypermetropia,  123 
in  myopia,  146 

R 

Railways,  regulations  for,  263 
Range  of  accommodation,  37 

convei-gence,  44 
Rays,  1 

incident,  7 

emergent,  7 
Recti,  internal,  146,  217 
Reflection,  2 

by  concave  surface,  3 

l)y  convex  surface,  6 

by  plane  surface,  2 
Refraction,  6,  22 

diminution  of,  128,  146 

estimation  of,  53 

index  of,  7 

by  lenses,  11 

by  i)]ane  surface,  6 

by  prisms,  8 

by  spherical  surface,  9 

liy  the  eye,  22 
Regulations  for  army,  261 

for  civil  service,  261 

for  English  railways,  263 

for  Indian  medical  service,  261 

for  navy,  261 

for  ijul/lic  works,  262 
Regular  astigmatism,  157 
Relative  accommodation,  40,  50 

convergence,  50 
Remotum  punctum,  2S,  35,  137 
in  emmetropia,  35 
in  hypermetropia,  38 
in  myopia,  137 
Retina,  21,  25,  147 
Retinal  asthenopia,  234 
Retinal  image,   size   of,  in    hyper- 
metroi)ia,  69 

in  myopia,  70 
Retinoscopy,  52,  66,  82 

cases  of,  102 

in  astigmatism,  96,  173 

in  liypernietropia,  128 

in  niy()])ia,  149 

mirror  for,  84 

o])li(iue  movements  in,  94 

plane  mirror  in,  81 

rate  of  movement  in,  93 
Rods  and  cones,  56,  Iti 
Rod  test,  16,  229 


INDEX 


273 


Scale  for  testing  deviation,  232 

Scheffler,  194 

Scheiner,  36,  64 

Scotomata,  144 

Secondaiy  changes  in  myopia,  147 

Shadows  in  retinoscopy,  82 

Shadow  test,  82 

Short  sight  (myopia),  27,  134 

Snellen,  56 

Spasm  of  accommodation,  197 

causes  of,  198 

symptoms  of,  198 

treatment  for,  199 
Spectacles  (see  also  glasses),  237 

for  aphakia,  133 

for  astigmatism,  176,  238 

for  hypermetropia,  129,  237 

for  myopia,  152,  237 

for  presbyopia,  190 

for  strabismus,  217 
Simple    hypermetropic   astigmatism, 
161 

myopic  astigmatism,  161 
Sqmnt,  see  strabismus,  200 
Staphyloma,  posterior,  138 
Stationary  myopia,  152 
Statistics  in  myopia,  142 
Stenopaic  slit,  181 

glasses,  157,  242 
Stereoscope,  221 
Strabismometer,  206 
Strabismus,  200 

alternating,  203 

angle  of,  207 

apparent,  200 

concomitant,  205 

constant,  206 

convergent,  124,  208 

divergent,  146,  208,  215 

monolateral,  20G 

paralytic,  200 

periodic,  206 

real,  201 

treatment  of,  217 
Sturm,  interval  of,  160 
Surfaces,  refracting,  of  the  eye,  22 
Sj^mptoms  of  astheiiopia,  225" 

astigmatism,  164 

hypermetropia,  124 


Symptoms  of  myopia,  144 
presbyopia,  190 


Table  for  presbj^opia,  191 

of  amplitude  of  accommodation, 
41 

of  angles  of  convergence,  49 

of  inches  and  dioptres,  260 

of  length  of  axial  line  in  hj^per- 
metropia,  122 
in  myopia,  138 
Tenotomy,  223 
Test  for  aphakia,  132 

clock-face,  170 

fan,  171 

letters.  Fray's,  168 

pin-hole,  54 

tj^jes,  for  near  vision,  265 
Jaeger,  61 
Snellen,  56 
Treatment  of  asthenopia,  227,  233,  236 

astigmatism,  176,  238 

hyiDermetropia,  129,  237 

myopia,  150,  237 

paralysis  of  accommodation,  197 

presbyopia,  190 

spasm  of  accommodation,  199 

strabismus,  217 
Tweedy's  optometer,  178 


Virtual  focus,  5 

images,  3,  19 

Vision,  acuteness  of,  29,  54 
binocular,  211,  219 
in  astigmatism,  165 
in  hypermetropia,  126 
in  myopia,  148 

Visual  angle,  29,  56 
axis,  201 

Vitreous,  148 

w 

Worth's  amblyoscope,  220 


Yellow  spot,  29,  56,  147 
Young,  157 


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